Take a pair of dumbbells and perform bicep curls in the following manner:

Curl both dumbbells up simultaneously.  Leaving one arm in the contracted position (the right one for example), lower the left arm and curl back to completion.  Now, slowly lower the right arm as the left arm stays in the contracted position.  Once you can no longer perform any more repetitions in good form, proceed with curls in a regular fashion.  You probably will only get 3-4 more reps.  Once you fatigue in that fashion, proceed bu alternating each dumbbell until muscular failure.  Again, you probably will only get a few more reps.

This approach can be used on a number of dumbbell exercises as well as machines that have independent movement arms.