I've done P90x and Chalean Extreme, so now I'm incorporating some of the Chalean philosophy on the P90X. Basically, I do the reps slowly (4 seconds on positive lift, 4 seconds on the negative). I've found that you don't need super heavy weights to max out on less reps if you do these 8-second reps. For instance, when I did my first round of P90X, I finished doing 12-15 reps of 20 pound curls. Now I still use the 20 pound weights but do the lifts slowly and can feel the burn between 8-10 reps. Does that make sense? You can do this until you can afford the heavier or adjustable dumbbells.

More...