Alan - I was referring more to prop firms, where you negotiate your "deal" and there is no set-in-stone rate. Retail brokers like MB I think are less flexible and I doubt will deviate from their posted commission rates.
About psychology, some (Bright seems to say this alot) propose just finding an edge and working it. If you are doing this and are confident in your strategy, psychology would not be a factor.
However, if you are arbitrarily selecting your trades, based on gut feel and what you think you know about the market, (which is what I'm sure most people do - judging by the posts on this board!), then psychology is everything. Then the odds are against you, since the human brain is wired to make poor trading decisions.. Fear/greed and all that..