Get thera-bands... basically you can go to about any doctor or physical therapist office and get them for free or very little cash. They come in different colors which indicate different strengths. You can tie them to the door knob, stand on them whatever to work the exact muscle you need. This is how I rehabbed my shoulder after rotator cuff surgery.

http://www.thera-band.com/store/index.php?CategoryID=11