http://www.ncbi.nlm.nih.gov/pubmed/24765078

So its not definitive but the gist of this is that it's NMDA rather than dopamine that causes the negative symptoms and they are suggesting that a glyT inhibitor which will increase NMDA signaling through the glycine regulatory site(ie without the neurotoxicity of glutamate stimulation)

Here is a roche trial for one inhibitor

Roche - Roche reports positive Phase II results with potential first-in-class treatment for the negative symptoms of schizophrenia

phase III is not going to be complete until 2015

However there are supplements which inhibit glyT like sarcosine

Here is a one source that is not mixed with aspartic acid ...

ProFrontal - a natural option to support NMDA Receptor function.

(I feel like a shill for these guys but its one of the more interesting supplements I 've seen)

Here is a discussion about this product apparently the effect is mild but worth it...
Any experience with D-serine/Sarcosine + NAC combined? - Schizophrenia Forums

The also talk about D-serine but that was abandoned awhile ago by psychiatry due to kidney toxicity so I would not go that route.

Anyway this is an active area of research so there is new stuff every day. Let us know if you find anything that works...