First, set the aileron endpoints to 120% in each direction. Move the aileron stick left and right to get to the two numbers.
[update: I thought I got this idea from Andreas, but he sent me an email asking about this and claiming never to have done it. Did I hear about it on RCG? Make it up out of whole cloth? Investigation forthcoming!]
Here's what Elevon mixing does, and how it's calculated.
Update: yes he did!