You are probably going to need some weight to do any good strengthening.
Hyperextentions. This is where you keep your legs straight and bend straight over. Go as far as you can go without losing a good arch in your back. You could definatly tightnen the lower back without weight, but it might take alot of them.
Not alot of great lower back workouts without atleast some dumbells.
Mainly the yoga poses that involve hyperextension or bending backwards; flexion of the lower back muscles; also many balance poses are useful.
I can never remember the names of the poses, and anyway there are often multiple names for the same pose. The best thing is to learn yoga in some form (a class, CD, book, etc.) and learn to "feel" the pose. Note whether it requires flexion/contraction of the erector spinae: if it does, then it'll probably help.