If you think about it too much it aint gonna happen. Abstaining from jerking off in otherwords setting yourself up for an easy erection can work in your favor. My advice don't overthink it, don't give yourself blue balls in an attempt to prep.
Just relax and if the mood strike it strikes.
I have had sex plenty of times on MDMA, and yes, I have cum, it takes a while and my best advice, mood lighting, Nudity, mirrors ( if you are comfy with yourself and your mate is too) and warm oil.
Warm up some coconut oil (Cold pressed virgin) it is a delightful natural lubricant that acutally smells good, tastes OK.... as long as you don't overdue it and is actually naturally antibacterial.
At around 75 degrees it is a solid and is kinda like well..... Vaseline you can eat, warm it up and it liquefies very easily. Take a spoonful and place it in a bowl microwave for a few seconds and it becomes viscous.
the warm oil on genetalia alone is enough to arouse me. Don't abstain for too long though E is funky, and could possibly cause Premature Ejac