I use this exercise in most of my ballet classes and pre-pointe classes. For younger dancers, I start with only 1st position, while the older ones I start adding sets. It is a great strengthener!
Position: 1st position facing the barre.
8 elevés in 1st position, 8 relevés in 1st position
8 elevés in 2nd position, 8 relevés in 2nd position
4 elevés on the weaker foot, 4 elevés on the stronger foot
repeat the weaker and stronger foot
repeat the weaker foot one more time, and try to balance for as long as possible.