How do you trim roots on a Deciduous Bonsai tree?

Late winter/early spring, before the buds start to put out new growth is the best time. You will need knowledge of repotting and soil mixes. The idea is to remove some of the old woody roots and shorten the newer feeder roots so that they will branch out. You have to totally remove the soil first, then when you repot it lay them flat across the new soil and apply more soil on top. Note: good bonsai soil is not potting mix or dirt, but rather a mix of grits, such as perlite, crushed lava, pumice, turface, akadama, gravel or very coarse sand, perhaps with some chopped conifer bark, all particles about 1/4" in size.

Look here for a lot of good information: http://www.bonsaisite.com/forums/