there was a write on up here i think, or at least a link to it about flushing. basically envolved takin the return line from the tranny and puttin it in a drain pan and have someone keep addin fluid while the other cycled through the gears until fluid turned pretty.
also, the method most, at least i am doing is sucking a qt out and refilling with clean a total of 3 or 4 times, running engine in between. will not get 100% of the oil out but sure as hell will help.