changing routines is a good thing..

have you tried..

1 warm up set of 6 reps at 60% of your working set
and
1 set to 20 reps (working set)

the working set is a weight that you can do 20 reps on ... if you get to 20 and you have not neded to really force the last 4... you need to increase the weight..

makes for a faster workout too...