It depends on the environment. For some things one just asks explicitly for less change. This works well with taxis. (Say there's an 7$ taxi ride, give a $10 and ask for $1 back). Another option in many contexts is to pay with the tip included and have it included in a way that shows it is obviously a tip based on the denominations in question (for example, if our taxi cost $9 and you hand them $11.25 it is obvious that you intended to tip $2.25)
Things to avoid: Do not give a large bill and say "keep the change" even if this is makes a generous tip or makes precisely the tip you want to give. The standard connotations of this are all negative (including but not limited to that you are rich, can't be bothered to think about change, can't be bothered to think about what is the right size tip, and don't really care much about the person you are tipping). If you only have a single bill it is better to tip less and get some small amount of change back than to say "keep the change." Another related thing to avoid is that when one is asking back for a specific amount of change, some people get annoyed if you ask for bills in specific denominations or specific coins. This seems to vary more by area and specific individual but it seems better to just avoid as an issue.
JoshuaZ:
Things to avoid: Do not give a large bill and say "keep the change" even if this is makes a generous tip or makes precisely the tip you want to give.
What is the exact source of this information? In a few years of living in (Anglophone) Canada, I've never heard of this. In fact, once you get the bill and put the money on the table, the waiter will often ask if you need any change. (Especially if the bill comes in that small folder and you close it over the money so he can't see how much you left when taking it.)
I am beginning to suspect that it is surprisingly common for intelligent, competent adults to somehow make it through the world for a few decades while missing some ordinary skill, like mailing a physical letter, folding a fitted sheet, depositing a check, or reading a bus schedule. Since these tasks are often presented atomically - or, worse, embedded implicitly into other instructions - and it is often possible to get around the need for them, this ignorance is not self-correcting. One can Google "how to deposit a check" and similar phrases, but the sorts of instructions that crop up are often misleading, rely on entangled and potentially similarly-deficient knowledge to be understandable, or are not so much instructions as they are tips and tricks and warnings for people who already know the basic procedure. Asking other people is more effective because they can respond to requests for clarification (and physically pointing at stuff is useful too), but embarrassing, since lacking these skills as an adult is stigmatized. (They are rarely even considered skills by people who have had them for a while.)
This seems like a bad situation. And - if I am correct and gaps like these are common - then it is something of a collective action problem to handle gap-filling without undue social drama. Supposedly, we're good at collective action problems, us rationalists, right? So I propose a thread for the purpose here, with the stipulation that all replies to gap announcements are to be constructive attempts at conveying the relevant procedural knowledge. No asking "how did you manage to be X years old without knowing that?" - if the gap-haver wishes to volunteer the information, that is fine, but asking is to be considered poor form.
(And yes, I have one. It's this: how in the world do people go about the supposedly atomic action of investing in the stock market? Here I am, sitting at my computer, and suppose I want a share of Apple - there isn't a button that says "Buy Our Stock" on their website. There goes my one idea. Where do I go and what do I do there?)