I had a pregnancy that turned high risk. What preparations should I make if I want to try again?

See a MFM. Be sure to see a board certified maternal fetal medicine pre conceptually prior to becoming pregnant for a formal consultation.
See your OB. What preparations you need to take depends on what exactly it was that made you high risk. In general, all women should ideally make preparations for pregnancy - eat a healthy diet, exercise regularly, start at a healthy weight, take a prenatal vitamin, etc. Additional specifics depend on your particular situation.