I was going to wait to post this for reasons, but realized that was pretty dumb when the difference of a few weeks could literally save people hundreds, if not thousands of collective dollars.

If you fly regularly (or at all), you may already know about this method of saving money.  The method is quite simple: instead of buying a round-trip ticket from the airline or reseller, you hunt down much cheaper one-way flights with layovers at your destination and/or your point of origin.  Skiplagged is a service that will do this automatically for you, and has been in the news recently because the creator was sued by United Airlines and Orbitz.  While Skiplagged will allow you to click-through to purchase the one-way ticket to your destination, they have broken or disabled the functionality of the redirect to the one-way ticket back (possibly in order to raise more funds for their legal defense).  However, finding the return flight manually is fairly easy as the provide all the information to filter for it on other websites (time, airline, etc).  I personally have benefited from this - I am flying to Texas from Southern California soon, and instead of a round-trip ticket which would cost me about $450, I spent ~$180 on two one-way tickets (with the return flight being the "layover" at my point-of-origin).  These are, perhaps, larger than usual savings; I think 20-25% is more common, but even then it's a fairly significant amount of money.

Relevant warnings by gwillen:

You should be EXTREMELY CAREFUL when using this strategy. It is, at a minimum, against airline policy.

If you have any kind of airline status or membership, and you do this too often, they will cancel it. If you try to do this on a round-trip ticket, they will cancel your return. If the airlines have any means of making your life difficult available to them, they WILL use it.

Obviously you also cannot check bags when using this strategy, since they will go to the wrong place (your ostensible, rather than your actual, destination.) This also means that if you have an overhead-sized carryon, and you board late and are forced to check it, your bag will NOT make it to your intended destination; it will go to the final destination marked on your ticket. If you try to argue about this, you run the risk of getting your ticket cancelled altogether, since you're violating airline policies by using a ticket in this way.

Additionally, you should do all of your airline/hotel/etc shopping using whatever private browsing mode your web browser has.  This will often let you purchase the exact same product for a cheaper price.

That is all.