I just found this page of exercises on a post by Ken Woodman at dance.net.
This is the thread where Ken discusses why these exercises are best.
I was glad to find this because I cringe when I see kids ask each other how to increase strength for pointe work and many seem to think "do a hundred releves a day" is more or less the solution.
I have to admit I have not taken up this issue thoroughly till now, and I will continue to add info when I find anything different. But, Ken (thank you), explains these well.