The most common cause of failures in ANY mag-fed semi-auto?
The magazine. This is especially true with ARs, which have had dozens of types of aftermarket mags made for them. Stick with USGI, Thermold, or Orlites, mainly USGI.
The next most common problem with ARs? The chamber. Colt, Bushmaster, DPMS, and NEW Armalite A2s and A4s use the larger M16 chamber with a longer throat. These rifles tend to be far more reliable than guns with tighter ".223" SAMMI-spec chambers. The barrel is the most important component of an AR.
Finally, use quality ammo. If in doubt, pick up some Federal American Eagle. If this ammo won't work in your rifle, your rifle is broken (or suffering from user error). Avoid military ammo (including Winchester Q3131) unless you have a rifle with the M16-spec chamber.
-Troy