How can I avoid infection if lymphedema?

Be careful. The national lymphedema network www.Lymphnet.Org recommends wearing protective gloves whenever doing any dirty work such as gardening, cleaning house, washing dishes etc. Good skin and nail care is essential. Go to the site and see all recommendations.
Meticulous skin care. The first step in avoiding infection is to get rid of the fluid. This is called decongestive therapy and includes wearing elastic compression stockings, using non-elastic circ-aid garments, sequential compression devices, lymphatic drainage/massage, diet and exercise (as dr. Brown said).This improves blood flow to the skin and decreases infection risk. Protect your skin as dr. Wagner said.
Learn lymph drainage. Learn how to do manual lymph drainage - lymphatic fluid does not use the heart as a pump. Lymph transport (or drainage) primarily depends upon the contraction of skeletal muscles—thus the importance of exercise. A body which is not moving will begin to stagnate in more ways than one! - begin streching the opposite side from affected side first - stagnation will cause infection.