During seated leg presses, what can I do to protect my knees?

If you're doing these at a gym you didn't mention which leg press machine you were using. The gym I go to has three different types. The most used is the seated incline press. If that's what you're using try using a little lighter weight. (you also didn't say how much you were pressing) As long as you don't feel any pain in the knee you should be OK with it. If there is a seated, straight out, leg press machine use that. It requires far less weight to accomplish the same results and doesn't put a lot of weight pressure on the knees. Before you do the leg presses do squats on a Smith Machine if you have one. That will warm up the knees for the presses. If you feel any pain either lower the weight or stop the movement.