here's some of the stuff i know about from personal experience...
1. i usually only mess with H, oxy or morphine and the best potentiator i've used so far is some good ol maryjane. (as far as increasing sedation and euphoria across the board)
2. any ethanolamine antihistamine will add to the sedation caused by opies, some more than others. diphenhydramine (benadryl) is the best.
3. i know for a fact cyclobenzaprine works good with oxy... i have a pretty high tolerance and was able to induce a powerful nod with double vision with 15mg flexeril and six 10/325 percs when it usually takes me 100mg oxy to get just a decent buzz.
4. benzos and alcohol seem to overshadow the opiate buzz. i recommend against these. *edit* i've since read that this "overshadowing" is most common in people not already used to/tolerant to these substances.
5. oxy after the peak of a good 2nd plateau dxm trip is heavenly imo.
6. a few shots of "nyquil cough" (doxylamine + dxm) does magical things when taken about 30-60min before a good hydrocodone dose. hint: exhilaration!!!
7. some people may not like it, but caffeine + oxy mixed together in the right amounts gave me a highly energetic, highly euphoric buzz. the oxy killed the nervous jittery aspect of the caffeine while still allowing a good boost in energy, and the caffeine killed the "out of it/don't talk to me/noddy effect" of the oxy. and it seemed that the overall euphoria was increased beyond what i would've felt from the oxy alone. it was good for getting f'd up and happy while still allowing me to be social and able to drive like a pro =)
(no, I DO NOT recommend driving under the influence, i'm confident in my own skills)
8. one time i went to an e.r. for withdrawals. they gave me clonidine. it helped a VERY TINY bit as far as helping me cope with the withdrawals. but once i got some more pills (morphs) I just had to see if it would work well with them. i ended up getting one of the most powerful nods i'd ever experienced. another friend i gave a few to said they screwed his vision just taking them alone. i'd say it works pretty nice but be careful with it. i'm kinda scared of the sh*t to be honest... it's a blood pressure med. it stops the production of norepinephrine and can be one hell of a sedative in doses as small as a fraction of a milligram.
9. i've also had pretty good results with ambien + morphine... although it wasn't a synergy type of effect... i could feel both drugs working independently but somehow it felt like the two drugs complimented each other pretty nicely.
10. that's about it... the only other thing i can think of is DON'T SMOKE YOUR OPIE, even if it's H. smoking is a *very* wasteful practice. those of you that smoke pills and claim to get high obviously don't understand that hcl's and other salts don't vaporize efficiently enough to get you high. most of your drug gets destroyed when you smoke your pills... and that high you experience is the result of burnt chemical fillers, wax, and broken down drug releasing strange (and probably harmful) chemicals that simply make you lightheaded cuz they're poisonous!
*edit* this is my first post & i now realize a lot of this shits already been covered. sorry bout that, just wanted to put in my 2 cents. also, i have just come up with a very accurate formula for converting sublingual buprenorphine to equivalent oral morphine or oral oxycodone doses (after alot of fukn reading). is there somewhere specific i should put it? 'til i find out, hit me up if you want it. =)