For a small investment you can buy some wrist exercisers. When using them, however, it is important that you use them correctly. If you just pump away at them, all you will do is build up strength, but it will limit flexibility, so the best way is close it and then flex the wrist, this will build both.
If you are a right handed player, then when pumping with the right hand, concentrate on squeezing with the two middle fingers, and on the left hand concentrate on the last three fingers, as these are the ones that consciously do the holding during the swing.
Using a squash ball in like manner can also be useful and it has the added advantage that you can take it with you wherever you go, well almost.