First of all, use the search engine. But, just because I always hated it when people told me that, I'll indulge your query...
Here is the ULTIMATE NDMA receptor antagonist thread.
The general rule of thumb is that you will want to take 30-90mg of DXM about 45 minutes before you dose your opioid of choice. It all depends on your body's relationship with DXM. Basically I recomend the highest dose you can tolerate without getting totally fucked up from the DXM, anywhere from 45-15 minutes before you dose, which in my case is about 100mg. The point it you want to get as much DXM in your system without letting its effect take over and mask the more desired opioid effects.
For best effect dedicate a day to it. I find I get the best tolerance reducing effects when I take 230-690mg of DXM in the late AM, then when it wears off 6-7 hours later dose my opioid. DXM in this way can actually help you avoid w/d symptoms, if take every day (that is 230mg+ DXM every day for 4-6 days).
Just to point out, there are two types of DXM: DXM HBr and DXM Polistirex. The above suggestions are for DXM HBr. I've never used the polistirex formulation in attempts at reducing my tolerance, but some argue against the use of the HBr formulation given health concerns. I don't see much of a need to too concerned with the neurotoxic effect of DXM at such low, therapeutic doses, but if you'd like to try the polistirex variety (found in Delsym as opposed to most other varieties, which are DXM HBr) you'll need to use more. So, if using DXM Polistirex, you'll need to use about 50-120mg, about double the HBr dose. That all said, I've never actually tried this, so it's just speculation.
IMHO I find potentiators, specifically Cimetidine/Tagamet to work better at increasing the opioid high than any tolerance reducing agent. But the tolerance reducing ability of drugs like DXM, especially for those who have a
really tolerance, can be quite amazing. If done right it can get you high like you haven't been since you started using...
With tolerance reducing agents you have to make sure you use them every time you dose your opioid, for them to continue having such a desired effect. This, with DXM or Ketamine or PCP, proves rather impractical. There are other NMDA receptor antagonists out there, some that work better than even DXM (which works really really well), but they're generally really hard to get.
Just read through that thread I linked to up above and you'll get all the info you need.
Next time, try searching
(p.s. MeDieViL = god)