The more hydrated your skin is, the better your tan will look. Lotion 2x a day with a spray tan friendly lotion (no mineral oil).
Avoid using any loofahs or washcloths during your showers. A clear, gel based body wash is best, or a hydrating bar soap (NOT DOVE!)
Avoid pools & hot tubs for long periods of time, long hot showers/baths, saunas, and excessive sweating. Your tan will not last very long if you plan on doing these things.
Skip intense workouts for 24 hours. After that, wear loose clothing and use a towel to pat sweat away.