How can I gain strength for ballet?

All movement from the legs comes from the core, so try to improve your core strength. Pilates is the best way to do that, but it's understandable if you don't have the money/time to take a few classes a week. DVDs can be bought, but try to go to at least one real class before you start with a video just to get a hands on feel.

If you can't take a pilates class or buy a DVD, you can do some exercises at home that you can find on the web. Of course there's the obvious crunches and sit-ups, but those only work one set of core muscles, so make sure to do exercises that work more than your abs. I really like "laterals" which are an exercise done in Horton modern, which work your obliques.

Glute strength is also important with holding your legs. Doing clamshells and leg lifts can help. An exercise I personally like is this:

Stand on all fours, with knees under hips and hands under shoulders. Make sure not to sway your back, and make sure to engage your core. Now lift your right knee out to the side at a 90 degree angle. Put it back to where you began and repeat until you feel the burn ;) make sure to do the left side too.

Also try lifting your leg with your arms, (making sure not to stick out your hip or anything--have correct alignment) and letting go and try to hold your leg. Think about using your core and glutes. This should help :)

Good luck!

-do leg lifts

-practise back stretches like the bridge

-practise over splits

-practise balancing on one leg whenever you can