Something I found that works great are resistance bands. I found them at Walmart with a 3-pack of the different strengths for around $5. When I worked, I travelled with my job and they were great because they could fit easily in a suitcase.

I just googled, and found this site that gives a really good demonstration of how they work:

http://www.sport-fitness-advisor.com...exercises.html

The only problem I encountered with them was that the strength in my hands is so bad, I had difficulty grasping the ends of them. What I really needed was to be able to wrap them around my hands several times, but the ones purchased at Wally world weren't long enough. I eventually found a place where I could buy a roll and cut to the length I needed.