Working your back
 

I was wondering what you guys do to work your backs. I've mostly been concentrating on my arms, and chest as far as upper body goes, and now I'd like to try to define my back a little. Also, are there any good ways to specifically increase the muscle in your lower back?

Straight Leg Barbell Deadlift (for lower back, glutius)
Dumbbell Row

check out: http://www.exrx.net/Lists/ExList/BackWt.html for more (way way more...)

As Arnold says, the best exercise for your lats is just standard pullups (with a comfortable wide grip). For beginners, which includes myself, he recommends doing 50 pullups when working out your back. Now I can't do 50 pullups at all so I use an assisted weight machine. You should also do bent over rows for your lats. Wide grip pull downs are also good for your lats, but pull ups should be the main exercise for those. For your lower back doing straight leg deadlifts are really good as well as good mornings, both of these will also work your hamstrings. Hyperextensions are also good for you lower back

50? Why bother? You'll build up your endurance, but you'll never build up your back's strength doing that.

Yes you will. Wide grips pullups work your lats. So doing any will work your back. 50 was just the amount he said to do. If you're weak it'll take you a while to do them. But as you get stronger it will take you less and less time to do all 50, so of course your back strength would increase.

Actually magua is half right. At first you will gain a little bit of strength doing them, but after your body gets used to it (which will happen quickly approximately 2-3 weeks) you will only be gaining endurance. In order to gain strength doing pullups, you would have to use a weight belt or ankle weights coupled with the pullups.

Ok yes I will agree with that. You will eventually need to add some weight to the exercise.

50 reps all at once? Or maybe 2 sets of 25? Or 5 sets of 10?

Whatever you can do, 50 may be much for a beginner, so just go for 5 set of 5 to start and work your way up.