Eat before hand, try dramamine (OTC also found in stores as Bonine sometimes), and have mints and chew minty gum.
As far as post nasal drip, if you're snorting it, just clear your nose out by snorting a bit of water about 30 minutes after.
Also to reduce itching, Dph, promethazine or whatever antihistamine suits you. Be careful though as they can increase drowsiness (which can be good, but it's dangerous so just be careful) and will affect each opiate/opioid differently.