most of this has been said already, but i'll add to it:
my roommate uses a foldable nylon bag when she shops. It's sort've like this
It folds up into a small pouch and then she keeps it in her purse and always has it with her. She's in the process of sewing me and our friends some. It's really sturdy and larger than the one i linked. (i found the one she uses at Claire's on a clearance rack)
We live in a dorm, so it's hard for us to compost, but the university has a garden with a compost pile off campus. We put all our compost stuff in the freezer until we can make it out to the garden to deposit it.
A really simple thing is to use all of the paper (write everywhere, print two sided) before you recycle it. I started doing this and was amazed at how much my paper use decreased.