A few things that have always helped me:
Know what you're looking for, and what it's worth. Don't just go looking for "something." This is the best way to find a good deal--because you know the current market, and what is a good/fair/poor price.
TALK TO THE SELLER. If you show genuine interest and knowledge they tend to be more interested in selling to YOU, and sometimes make a better deal. (I'm 21, I usually get brushed off as some un-knowledgeable punk kid).
Cash talks. I've made some sweet deals making it look like "but this is all i have, will you take it" type strategies.
(I usually have certain amounts of cash in each pocket, wallet, etc. This way I've only got 18 dollars, or 45 dollars, or 312 dollars,
)
If you don't see what you're looking for at a table, but see similar items, ask (especially in the mil-surp world). Many times dealers have extra stock tucked behind the table--or the nice one they aren't quite sure if they want to sell, yet.
Be considerate of the sellers. Don't just grab the item off the counter-show some interest and ask. Many sellers get very agitated by people who just run up to the table, grab a rifle, and start tinkering with it.
DEFINITELY ask before you take anything apart or dry fire.
If a seller isn't willing to drop the price, use phrases like "that price includes tax, right?" (not always applicable) or "if I buy this case of ammo, will you throw in a package of stripper clips? (or patches, oil, etc). Or, my favorite, this handgun comes with
three magazines for this price, right? Sometimes they're more willing to give you a free $15 item rather than take $10-15 off the price. You still get a good deal, and it's less money out of their pocket (mark-up).
Never get impatient, or buy the first thing you see. Undoubtedly, you'll find a better deal 3 tables down. Watch the reactions of other potential buyers--if someone more knowledgable passes a "good deal" you should too...it's probably junk.
If a seller isn't willing to let you partially dis-assemble, or has shady responses to your questions pass the item up. There are a lot of people out there that give junk a once over to look "nice" and try and make a buck...trust your instincts.