this is something I get my students to do.
It definetely helps to have someone to help you.. fold your fabric selvage to selvage across the 45 width. One person take one end, and you the other end. HOld the fabric piece in front of you both. A hand is placed on the folded part, and the other on the selvage part... Both using your right hands first, give the fabric a little tug so that it is stretching a little across the bias.. Do the same thing then with your left hand. I do this twice with each hand.. then I reposition it and I find it folds much better most of the time.
Just remember not to stretch the bias too hard.
Hope this helps