I have four Glock pistols. Apart from being almost perfect from the factory, here's what I modify my Glocks with:
1. Extended slide release-this one is a million times better than stock, it takes little force to engage it properly but it just works alot better than the flat, square slide lock that is OEM standard.
2. Extended mag release, again much better than the original, don't worry, like others have said on this site that it gets in the way and you are more prone to mag drops etc. I ran a Glock 22 in IPSC for four years and never once unitentionally locked the slide or dropped a mag at a bad time. Not once.
3. NY+ trigger with a 3.5" connector. Probably the best way to smooth out the trigger. The pull is consistant and breaks at about 5-6pounds. The standard spring makes the trigger hard to deal with, the NY triggers are different than springs so it really smooths things out.
All four of my Glocks have these modifications, it is very cheap for parts and my Glocks run flawlessly in every type of environment. As mentioned Glockparts.com for parts. Can't beat the selection, service and economy for OEM Glock parts.