Most people use glycerol starting concentration from 20 to 60% in BHI. Higher concentration of glycerol also cause toxicity. But with anaerobes you will have to extra careful. Freeze drying with a stabilizer is a best method.

Apart from glycerol, you can also consider to add trehalose as protectant.

You can use cryoprotectant media consisting 12% [w/v] skim milk powder, 1% [v/v] dimethyl sulfoxide, and 1% [v/v] glycerol.                                                                            

You can also use 0.4 mM polyvinyl pyrrolidine, instead of glycerol or DMSO, the results were found superior while using polyvinyl pyrrolidine 

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC242782/

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3266037/