Diemde, like liftnlady said, this part sort of happens by itself as you progress through your training - you're going to get to a point where you plateau - you won't be able to lift any heavier, but you won't have any DOMS either. At this point, you'll do things like dropping the weight down an increment and doing longer sets, or leaving the weight alone but doing reps to failure (lactic acid training - stimulates growth hormone), or lifting a heavier weight in many VERY short reps (like 8 sets of 3), or....

You get the drift.

And I stretch like you do - after I lift, or sometimes, all through my workout. I don't do any separate stretching programme either.