For buying:
HAGGLE!
It's a lost art. Haggle for everything that you're not buying in a shop, and sometimes even then if the feeling's right. Also, keep your eye out for deals and bargains in places you wouldn't expect. Lots of people have guns, but many of them don't place the value that you or I would. I've found some nice collector-grade stuff for great prices from people who own guns, but only own them as shooters. Thus, if that Luger (or whatever) isn't getting shot, it may as well go away, and you end up with a real bargain.
Check the market trends. People are buying black rifles? Buy collectibles. People are buying collectibles? Sell your collectibles and buy some black rifles. Stay away from the current trend, and you can buy stuff on the cheap and even make money when the market turns.
Bang-for-buck. This takes research. Learn what's what, know what the price should be when you see it, and jump if the time is right. I've been kicking myself for not buying an Egyptian RPK for $300 two years ago. Sporterized mil-surps often shoot just as well as their collectible cousins. Cz82's reign supreme in handguns.
For gear:
I've found Tagua leather products to be wonderfully made holsters for a modest price.
Airsoft gear works great for tacti-cool applications where you don't need the heavy duty-ness of the real gear, and it's far cheaper.
For Maintenance:
"free" toothbrushes at the dentist? Great for cleaning hard to reach areas.
Motor oil makes great lube.
Hot water is close to free and evaporates from metal quickly; it's primarily a black powder trick, but it cleans up smokeless guns too and on the cheap.
Learn to do some basic gunsmithing: polishing, brazing, fabricating, bluing or parkerizing. You can take a class or two at the local community college for the price of some jack-leg gunsmith work at the local metal-butcher. That way you can save money, and if it's broke when you're done, you don't have to be mad at anyone but yourself. If it's better when you're done, you get to break it later trying to make it "perfect."