One of Smokin' Joe Frazier's pieces of advice was to stay off of weightlifting if you were serious about it, and if you're going to weightlift, just to benchpress. There's really a lot of controversy over whether boxers should lift weights or not. I heard that alot of the bulking lifts add mass to your arms that potentially slow down the punch speed, when the power of a punch really depends on its speed and how much body weight you put behind it.
This is the workout Joe Frazier did when he knocked down Ali in rd. 15
First, all the heavy bag/speed bag/sparring/focus mitts/medicine ball/etc.
then calisthenics:
25 pushups
10 dips (or pullups)
25 pushups
10 dips
25 pushups
10 dips
Looks easy at first sight, but after putting your upperbody through all the bag and mitt work, it's pretty damn tiring.
I had a trainer at my old school that used to box, and he suggested doing a lot of explosive exercises in the weight room. Power cleans, hand cleans, clean and jerks, front drops, and things like that.