I use plain ol' Sculpey, and sometimes, Premo. They're easy to shape for someone who can't sculpt like me.
I don't use any sculpting stuff for backpacks and weapons, unless I want to fill in holes or seams. For those I cut, sand, glue, and shave existing parts.
When I want to add a feature to a figure, like miscellaneous armor, trinkets, or bulk, I've begun using Sculpey in tandem with super glue. Here's the process:
1 - Apply a very small dab of super glue to the surface I'll be adding sculpey to.
2 - Use a small patch of sculpey (or other compound) to cover the glue.
3 - Add more compound and sculpt it to your heart's delight.
4 - When you have the desired shape made, seal it with enough super glue to completely cover the compound. Don't worry if it looks like a big, shiny, clear gob over the compound.
5 - Let it dry completely. Sometimes this takes a couple of hours.
6 - Sand down the dry, sealed compound to the desired shape. This is where the major detail work happens.
7 - Paint it, let dry, seal with Testors Dull-Cote.
You can substitute ingredients, but I've found super glue really hardens the sculpey to a very durable state. As I said, I don't use it for anything I want to remain flexible. For flexible stuff like belts, suspenders, and other misc. straps, I use thin rubber pieces. Generally I cannibalize Joe, SW, Elite Force, and Lego toys to get the rubber or plastic.
Hope this helps!