What are some good exercises to do to prepare for pointe work?

Barre and center

To put it less succinctly than citibob did, readiness for pointe work is determined by the level the student has reached in her (or his) regular class work. The most efficient way to achieve this level is to take classes. Also, since strength of the body overall is necessary to start pointe work, and not just strength in any particular part of the body, no special exercises are needed. Want to be more ready? Take more classes.

I would agree with the above posters and add that some good foot things that you can do include making sur le cou de pied, continuously from 5th position in a pas de chaval action to front and/or side, using the bottom of the foot for this and not just picking up your leg from the thigh.

I'd also recommend observing (or taking) a beginning level pointe class (not on pointe but in flat ballet slippers) and learning some of these exercises, many of which are specifically designed to strengthen and gain control not only of feet but also of the inner leg, balance, and with finesse.