There is an interesting thread on this on thepaceline: http://forums.thepaceline.net/showthread.php?t=128544

In the 3rd post, there is a link to a thread on some other forum for this as well, with some interesting tips:
http://weightweenies.starbike.com/fo...hp?f=3&t=53264

I too have had problems with getting a good blend (or any blend at all, really), but haven't been anal enough to spend the amount of time on it that is really required.