Armature advice for the hip region I'm using a basic armature: a couple of bones for the hand, a bone for the abdomen, chest, neck, head, clavicle, arm, forearm, thigh, shin, ankle, foot and toes. I'm use poles and targets for the arm and leg. 

This basic setup works fine for walking animations, attacking, tree chopping etc. but as soon as I attempt an animation where the model is picking something up or crouching, it looks awful. The buttocks completely disappear and the abdomen is pushed in by the thighs. I've tried to weight paint my way out of this but I suspect I need more bones around the hip region to stop this from happening.

Does anyone know any good resources for improving armatures? Keep in mind that this is for a game so I'm aiming for minimal bones. I can post my blender scene on pasteall if that would help.
No, you don't need to add extra bones. You may need to rearrange some of the existing bones.

Remember, when something doesn't work like real life, the best thing you can do is try to mimic real life.

Here's a picture I drew, with proper weights on the leg and a diagram of how the real bone structure is.



Notice how the effects of the leg bone has are very little on the glutes. This will help with crouching as the but won't have to stretch out in order to reach a full squat. Above the hip joint there should be little to no weight, except a little on the abdominal muscles, which need to suck in a little in order for the leg to not collide with them. 

I think this should solve most of your problems; it always works for me.