Any of the above can streak and ruin a new tan. If you want to go darker, you can repeat the process the day after you tan. Go easy on the exfoliation the second time around so that you don't remove your first layer. Once your tan is set, keep yourself well moisturized. The longer it takes the skin to flake off, the longer your tan will last.
Most bottled tans begin to fade after a few days.
You'll need to reapply once or twice a week to keep your color.