There's two ways to do this, electronically, and mechanically. I'll briefly describe each.

One way to do this electronically would be to set the ailerons to travel the same up and down and then use the end point adjustment function to limit the down. You would make these adjustments on high rates and then set low rates to be a percentage of the max. The disadvantage to doing this is that you are not allowing the servo to travel its full range so you lose some sensitivity.

The mechanical method relies on just offsetting each servo arm so it's not square to the control rod. Rather than trying to describe it with words, here's a link to a thread that contains one of MinnFlyer's famous pictures
http://www.rcuniverse.com/forum/fb.asp?m=1706117

I recommend the mechanical choice, it's simple, and allows you to use the entire servo range.