As spacejunk says, your body naturally regulates the amount of neurotransmitters within the cells of your brain. Think of each of these cells as a water tank connected to a tap above a sink. The brain tries to keep the 'right' amount of water in the sink by regulating how much the tap is turned on / off and by making the drain in the sink bigger / smaller. MDMA causes the brain to flood itself with serotonin (as well as affecting other chems) so the analogy is the sink is flooded with water and the tank drained.
The trouble comes once the MDMA wears off and the body starts regulating again. The analogy here is it's trying to get back to having the 'right' amount of water in the sink, but you've just drained the tank. Hence the low feeling a few days after a big night. It's that common that people call it either 'Blue Monday' or 'Suicide Tuesday'.
So, what are some strategies to help 'fill' your 'tank'. The problem is these neurotransmitters are not something you can just take and to make them from what you eat your body goes through several stages. My suggestion is to take longer breaks between flipping, eat well, get plenty of rest and resist the urge to 'top up' on pills (you won't ever get back to where you were). Supplement wise you can try taking things to help 'prime the pump' to refill your 'tank'. I have used all of the following with different levels of success. If you search BL there will be a wealth of info under the topics of 'pre-loading' and 'post-loading'. As a start, you can research L-Tyrosine, 5HTP, L-Dopa and GABA.
Good luck and party safe.
