How I have learned to do it, is to keep the same routine for 6 months and track all of your lifts. The goal is to add reps or weight every single time you go. So if I'm doing 3 sets of 12, if I get 14 reps on the final set, I add 5 lbs the next time I lift.

The goal is to get up to 3x12 with the new weight, and then add weight, rince and repeat. So even though I'm doing the same routine, I'm always pushing myself.