Here are some really good pointe strengthening exercises.
1. Push the Box. Sit on the floor with your legs strait in front of you. Place a box at your feet. Push the box away from you with your toes as far as you can. Then, reach over your legs and grab the box and bring it closer. This will help strengthen your ankles and has a bonus of a leg stretch.
2. Alphabet Toes. When sitting down at the table, or watching TV, write the alphabet with your toes. You can do Manuscript or Cursive, whichever is easier for you. This will strengthen your ankles.
3. Step, Plie, Step, Plie. This is a pre-pointe exercise straight from the studio. Stand in "6th" position on releve. Take a step forward. While still in releve, bend your knees in plie. Straighten. Take another step forward and plie. Do this down the hallway or even on your way to the refrigerator. You are on releve the entire time.
4. Releves are excellent for gaining strength in calves and ankles. Stand on a stair step with most of your foot hanging off of the edge. Stand up on releve and come down slowly. When you come down, you will go past flatfoot and will stretch out your calves. Releve 20 times or so, and work your way up to more as you feel comfortable.
5. Roll the Can. While sitting down at dinner, or watching TV, find a can of food and roll your foot on it. This will help improve you arch, and give you a better pointe. For you "Prima Crafters", you can Mod Podge scrapbook paper onto a can of beets or something no one is going to eat, and decorate however you want! (I'll come up with a tutorial for ya'll ;) )