What should I take after a steroid course of testostorone and deca to avoid side effects?

Daignosis needed. Why were these medications prescribed? Are they short term or long time? This is important. In order to advise how to avoid side effects we need to know what they were given for. There may be alternate therapy that would not require these meds. It depends what your risks are, what your co-morbidities (other medical problems) are. We need a diagnosis - ask your doctor.
Which side effects? Which side effects do you hope to avoid? If you want to maintain fertility, then neither Testosterone nor Deca-Durabolin are recommended as both will suppress sperm production. Side effects can also occur during a cycle, not just afterwards. Heart attack, stroke, elevated blood pressure, hair loss, prostate enlargement, gynecomastia, etc. These complications can be avoided by close monitoring. .