Hehe, I can always tell when DD puts on more weight because my back is sore for a week! Granted, she's tiny, 10 months and 16lbs, but so am I.
Lift with your knees. Honestly though, invest in a good carrier (I love my Ergo and Moby) that will help distribute his weight when you have to carry him for long periods of time, or even just around the house.
The only tips I have are good body mechanics. Things I learned in nursing school haha. Like bend your knees to pick up, don't just bend forward. Keep him (or other heavy objects) close to your body. Never twist around, if you're going to change directions then pick up your feet and move them into that direction.
Body Mechanics are key, for sure. NB has great advice on that. I've also used a few yoga with baby type dvd's that will help strengthen you using exercises with your monster man. Two birds with one stone (playing with him and working on your own body).
As he got bigger there were certain things I just gradually got stronger on (holding him on one hip for long periods of time) and some that repetition aggravated (like my left hand holding up his feet to change a diaper tens of times a day).
(Just thought I'd throw one more thing in parenthesis at the end, since I appear to be married to it in this thread).