I think people who are not made happier by having things either have the wrong things, or have them incorrectly.  Here is how I get the most out of my stuff.

Money doesn't buy happiness.  If you want to try throwing money at the problem anyway, you should buy experiences like vacations or services, rather than purchasing objects.  If you have to buy objects, they should be absolute and not positional goods; positional goods just put you on a treadmill and you're never going to catch up.

Supposedly.

I think getting value out of spending money, owning objects, and having positional goods are all three of them skills, that people often don't have naturally but can develop.  I'm going to focus mostly on the middle skill: how to have things correctly¹.

1. Obtain more-correct things in the first place.

If you and I are personal friends, you probably know that I have weird gift-receiving protocols.  This is partly because I hate surprises.  But it's also because I don't want to have incorrect things, cluttering my space, generating guilt because they're gifts that I never use, and generally having high opportunity cost because the giver could have gotten me something else.

This problem isn't only with gifts.  People get themselves incorrect things all the time - seduced by marketing, seized by impulse, or too hurried to think about which of several choices is the best one for their wants and needs.  I have some incorrect clothes, which I got because I was sick of shopping and needed a new pair of pants even if it was terrible; as soon as I found better pants (or whatever) those clothes were never worn again and now they're just waiting for my next haul to Goodwill.  I bet a lot of people have incorrect printers, mostly because printers in general are evil, but partly because it's irritating and dull to investigate them ahead of time.  Cars may also tend to fall into this category, with a lot of people neutral or ambivalent about their self-selected objects that cost thousands of dollars.

If you are not currently living in a cluttered space, or feeling guilty about not using your objects enough, or tending to dislike the things that you have, or finding yourself wanting things that you "can't" get because you already have an inferior item in the same reference class, or just buying too many not-strict-necessities than is appropriate for your budget - then this might not be a step you need to focus on.  If you have objects you don't like (not just aren't getting a lot out of, that's for later steps, but actually dislike) then you might need to change your thresholds for object-acquisition.

This doesn't mean something stodgy like "before you get something, think carefully about whether you will actually use and enjoy it, using outside view information about items in this reference class".  Or, well, it can mean that, but that's not the only criterion!  You can also increase the amount of sheer emotional want that you allow to move you to action - wait until you more-than-idly desire it.  If I were good at math, I would try to operationalize this as some sort of formula, but suffice it to say that the cost of the object (in money, but also social capital and storage space and inconvenience and whatnot) should interact with how much you just-plain-want-it and also with how much use you will likely get out of it.

Speaking of how much use you get out of it...

2. Find excuses to use your stuff.

I have a cloak.  It cost me about $80 on Etsy.  It is custom made, and reversible between black and gray, and made out of my favorite fabric, and falls all the way to the floor from my shoulders, and has a hood so deep that I can hide in it if I want.  If I run while I wear it, it swoops out behind me.  It's soft and warm but not too warm.  I like my cloak.

I also have sweaters.  They didn't cost me anywhere near $80, not a one of them.

When it's chilly, I reach for the cloak first.

I'm playing a game with my brain: I will let it make me spend $80 on a cloak, if it will produce enough impetus towards cloak-wearing and cloak-enjoying that I actually get $80 of value out of it.  If it can't follow through, then I later trust its wants less ("last time I bought something like this, it just hung in my closet forever and I only pulled it out on Halloween!"), and then it doesn't get to make me buy any more cloaklike objects, which it really wants to be able to do.  (I don't know if everyone's brain is wired to play this sort of game, but if yours is, it's worth doing.)  My brain is doing a very nice job of helping me enjoy my cloak.  Eventually I may let it buy another cloak in a different pair of colors, if it demonstrates that it really can keep this up long-term.

People sometimes treat not using their stuff like something that happens to them.  "I never wound up using it."  "It turned out that I just left it in the basement."  This is silly.  If I'm going to use my cloak - or my miniature cheesecake pan or my snazzy letter opener - then this is because at some point I will decide to put on my cloak, make miniature cheesecakes, or open letters with my snazzy dedicated device instead of my nail file.  You know, on purpose.

Sure, some things seem to prompt you to use them more easily.  If you get a new video game, and you really like it, it's probably not going turn out that you never think to play it.  If you get a cat or something sufficiently autonomous like that, you will know if you are not paying it sufficient attention.

But if you get a muffin tin and you have no pre-installed prompts for "I could make muffins" because that impulse was extinguished due to lack of muffin tin, it will be easy to ignore.  You're going to need to train yourself to think of muffins as a makeable thing.  And you can train yourself to do that!  Put the muffins on your to-do list.  Lead your friends to expect baked goods.  Preheat the oven and leave a stick of butter out to soften so you're committed.  If that doesn't sound appealing to you - if you don't want to bake muffins - then you shouldn't have acquired a muffin tin.

Speaking of your friends...

3. Invite others to benefit from your thing.

I've got a pet snake.  Six days of the week, she is just my pet snake.  On Saturdays, during my famous dinner parties at which the Illuminati congregate, I often pass her around to interested visitors, too.  The dinner parties themselves allow my friends to benefit from my stuff, too - kitchen implements and appliances and the very table at which my guests sit.  It would be less useful to own a stand mixer or a giant wok if I only ever cooked for myself.  It would be less pleasant to have a pet snake if I had no chance to share her.  It would be less important to have pretty clothes if no one ever saw me wearing them.

You're a social ape.  If you're trying to get more out of something, an obvious first hypothesis to test is to see if adding other social apes helps:

• Loan your stuff out.  (People seem to acquire physical copies of books for this motivation; it is good.  Do more of that.)
• Acquire more stuff that can be used cooperatively.  (Own games you like, for instance.)
• Find creative ways to use stuff cooperatively where it was not intended.
• Tell people stories about your stuff, if you have interesting stories about it.
• Fetch it when it is a useful tool for someone else's task.
• Accept compliments on your stuff gleefully.  Let people have experiences of your stuff so that they will produce same.

Also, circling back to the bit about gifts: I bet you own some gifts.  Use them as excuses to think about who gave them to you!  My grandmother got me my blender, my mom made me my purse, my best friend gave me the entire signed Fablehaven series.  Interacting with those objects now produces extra warmfuzzies if I take the extra cognitive step.

Speaking of how you go about experiencing your stuff...

4. Turn stuff into experiences via the senses.

Remember my cloak?  It's made of flannel, so it's nice to pet; it's fun to swoosh it about.  Remember my snake?  She feels nifty and cool and smooth, and she looks pretty, and I get to watch her swallow a mouse once a week if I care to stick around to supervise.  I get candy from Trader Joe's because it tastes good and music that I like because it sounds good.  If you never look at your stuff or touch it or taste it or whatever is appropriate for the type of stuff, you might not be having it correctly.  (Raise your hand if you have chachkas on your shelves that you don't actually look at.)

Caveat: Some purely instrumental tools can be had correctly without this - I don't directly experience my Dustbuster with much enthusiasm, just the cleanliness that I can use it to create.  Although nothing prevents you from directly enjoying a particularly nice tool either - I have spatulas I am fond of.

And of course if you choose to follow the standard advice about purchasing experiences in a more standard way, you can still use stuff there.  You will have more fun camping if you have decent camping gear; you will have more fun at the beach if you have suitable beach things; you will have more fun in the south of France if you have travel guides and phrasebooks that you like.

¹It's an optional skill.  You could neglect it in favor of others, and depending on your own talents and values, this could be higher-leverage than learning to have things correctly.  But I bet the following steps will be improvements for some people.