I think the only thing antihistamines will help with in regards to aMT is to help reduce swelling or puffiness of the eyes/face that serotonin agonists can sometimes cause, or at least aMT does to me every time.
Personally I don't use DMH or DPH though; these can have undesirable side effects (a single dose likely won't trigger anything but you never know w/ drug-drug interactions) and can make you tired. Try something like allegra if you get the puffy face or have allergies.
As for nausea and vomiting, the best aid is #1 not having lots of food in your stomach and intestines to begin with (i.e., fast as much as you can in the 24-48 hours before the trip) and #2 ginger - either fresh powder or the fresh root, capsule and eat the powder or make tea from the root.
Some of the OTC meds can help too, pepto bismol works pretty well for me, antacids, and loperamide maybe, sometimes I feel like it helps others not.