Chin-up negatives are a good movement to improve strength, even for those that can't do a chin-up. Chin-up negatives consist only of the second part of the chin-up movement: start at the top position of a chin-up, and slowly lower yourself down to the bottom in a five-second count.
Do three reps at the end of each set of your usual chin-up work, or if you can't do a chin-up then start with three sets of five negatives and each week try to add a rep each set. Retest your chin-ups every four weeks and note the improvements.
Test your biceps at Men's Health
Chin-Ups on Wikipedia
How to Do a Negative Pull Up on YouTube