I am just starting working with foam for RC planes but have done lots of work with foam mockups in my job. Usually this is with foam core board (with a paper skin on both sides, usually 1/4" thick).
Having done several searches on this and other sites - and not found much basic information on forming things out of foam sheets - I thought I might start my own "resource". Hopefully others, with more airplane experience, will add their methods.
The image is a sketch of the most basic corners I have used. Although they are drawn with foamcore (paper skinned) in mind, non-skinned foam would be worked the same way, although packing tape, etc. would help.
Probably the most interesting method is the last one shown, using multiple diagional slices to make a gentle curved shape. Try it, you'll like it.
Allan