I was searching on
Google Scholar for you and thought it was really funny that one of the first things that came up was a Bluelight thread! I really didn't think Google considered us to be that scholarly
Studies on NMDA antagonists preventing or slowing the increase of amphetamine tolerance are actually difficult to find. Perhaps that's why you're asking here? Many of the studies I was able to find are actually about how NMDA antagonists can prevent behavioural
sensitization to amphetamines (in animals at least).
But I don't think that preventing sensitization and preventing tolerance are mutually exclusive.
Just one example:
There is one on magnesium increasing the effects of amphetamines, but it doesn't say anything about tolerance:
One on preventing
cocaine tolerance:
Sorry I haven't been able to find anything better for you yet.
Epsilon Alpha - Do any of the threads on the topic in Advanced Drug Discussion have links to or quotes from relevant studies? They were too long for me to read through