Sorry to be a party-pooper but I'm not a fan of polydrug use. The reason for this being that I got very sick from mixing substances on New Year's Eve 1999/2000.
Two weeks after NYE, I started fainting and losing consciousness, at random, for no apparent reason. I was too frightened to drive in case I blacked out behind the wheel.
I went to see my GP (who was aware of my drug use) and told her everything I'd taken in the previous month. On NYE alone, I had 2.5 pills [MDMA + 2CB], a line of speed, 3 dexxies, some coke and lots and lots of nitrous. Now that seems to be a pretty standard evening for some people, but according to my GP it was lucky that I wasn't suffering any worse symptoms. After several tests we discovered that there had been no permanent damage done.
My GP doesn't condone my taking drugs, but supports a harm-minimisation approach in the hope that I do less damage to my body than someone who isn't aware of long-term consequences.
On her advice, I am now sticking to single drug-use. It doesn't matter if I'm not experiencing the same effects as someone who's had a cocktail of drugs. Putting my brain at risk of permanent damage just isn't worth the "fun" of a few hours mixing substances.