for me the trick is to think about it beforehand (usually means coming up with the justification to buy one), save the money, and jump on the first one you find that meets the price point. I was probably a year away from buying one when I found an upper set up almost exactly like my 5.56 upper for really cheap on gun broker and immedately "bought it now". The guy even threw in a bolt carrier and charging handle for a good price, and when I got it, found out it was ramped (that meant that the 9mm hammer I bought was not useful, but that was easy to sell, and the ramping saved me $40). Ended up with a fancier setup for about $800, and I did not even end up with any extraneous stock parts cluttering up my house.
Until then, the parts that are hard to get cheap is the barrel and mag block, and I jumped on the latter when I saw a good deal. Looking at forums late at night works pretty good.
Admittedly, good deals are less easy to come by as the market is not as big and the people who want them really want them. I have the advantage of knowing a bunch of gun yuppies who are constantly buying stuff then selling to get a new toy.
also, you need to do a good amount of research to make sure your parts are all compatible.