Simple tip:
I goes without saying that keeping EVERYTHING goes along way in helping with kitbashing or customizing. Items from past lines, like Spytroops or Valor vs Venom can be a BIG help in creating new and interesting customs/kitbashes. Keep your eyes open for stuff that goes on sale really keep, or is given away by friends, family and other collectors. Since the earlier lines have fallen out of favour with some collectors, you might be able to get a bunch of stuff for pennies on the dollar.
Also, don't restrict yourself to just GIJOE brand items. Other 1/18th scale figure lines can have a variety of gear, from weapons to helmets to other items that can work seamlessly with GIJOE customs.
Another simple tip: a customizer's best "expedient" friend: poster putty.
I love this stuff, because a small blob can do so much. Have a neat looking helmet, that looks right, but fits too big on the figure's head? No problem, a small blob of putty on the crown of the head and the helmet will sit secure for as long as you want.
Pistol falling out of a holster all the time? Same dealio--small blob of putty on the barrel and slide the weapon in. It'll hold fast.
Removal is easy: if the putty sticks or gets caught in rough textures, use another blob of putty to pull the trapped stuff away. It grabs itself very well.
It can also be used to add another attachment point for feet on stands.