Stretch the antagonist muscle!
From t-mag:
The Push This, Pull That Principle
Most people do successive sets of a particular exercise. They pump out 8 reps on the bench press, pick their nose for 60 seconds, and then do another 8 reps. This goes on until the muscle is properly pumped or the nose is properly picked.
However, if you do a set of bench press, rest briefly, and then do a set of an exercise that works the opposing muscle group — like a set of barbell rows, for instance — you get much better recuperation. This enables you to perform much better on the second set of bench press than you would have otherwise.
Similarly, if you stretch the opposing muscle group in-between sets of an exercise for the antagonistic muscle group, you’ll also get increased performance. Again, using bench press as an example, if you stretched your lats by pulling on an immobile post or bar, you’d "prime" your pecs for their next set of bench. With any luck, you could experience an increase in strength anywhere from 2 to 3%.
We’re not 100% sure why this happens, but you can bet it’s because of the nervous system.