Get your dancer's guide that teaches how releasing muscle tension every day, increases strength for dancing in pointe shoes.
If you have just started dancing in pointe shoes, or are preparing to, here is one thing you can do to strengthen your feet.
You use the floor as resistance in every tendu and degage you do.
You PRESS into the floor with the sole of your foot, every time you leave fifth or first position and stretch your foot into a tendu, degage, grande battement, or go through first in your ronde de jambe a terre.
You also PRESS through the metatarsals on to full pointe to the end of your tendu, degage or grande battement.
This will strengthen the sole of the foot, and the muscles under the metatarsals (toes) which will help you in pointe work.
It will also help you in jumps, lowering into a demi-plie with that cat-like quality, and avoid many injuries from landing well, when you are on a less-sprung floor.
It is important for you to realize that all your basic ballet work is preparation for your work in pointe shoes. Every small movement from a closed position to an open position is a chance to strengthen the sole of the foot. Pressure into the floor is like the resistance of a heavy weight. The harder you press, the heavier the weight.
Relaxing the foot muscles is important. You can roll your foot over a tennis ball, but better yet use a rubber ball that has a little give.
Here's a "pinkie ball" stretch I learned from material by Deborah Vogel about ballet stretches. With a soft rubber ball, kneel down on the floor and put the ball under one of your legs, under the shin. Let your weight press into the ball, move it down inch by inch, and it will relax the tibial muscles. Go all the way down to the ankle area, kneading and releasing muscle tension.
Then, put the ball under the top of the metatarsal area, and pressing into it, you will get a stretch down the top of the foot and over the ankle, increasing the curve of your point.
Remember, muscles are stronger and better toned when they get stretched properly, and relaxed fully, every day.
Pointe shoes make noise, so for that, among other reasons, the more control the better.
The upper body posture and control is part of all of this too - but the feet is where you meet the stage. I hope this helps.
For more exercises about how to release muscle tension, get yourself a dancer's guide.