One thing I have been doing are bicep curls. I mean, a lot of them. I start out by doing 20, then 15, 10, and finally 5. I alternate my arms after each set, and by the end I’ve got nothing left. Another good idea is to change it up a bit if you start to plateau. Come up with some different set ideas, or alternate the reps. 20, 10, 5, 15.