Follow your doctor's directions for the pre-surgery diet. Start getting used to drinking a lot of liquid. Start walking if you haven't already. If you aren't a naturally slow eater, start trying to train yourself to eat very slowly and chew everything very well. Look at what life will be like after surgery and try to emulate that and start building habits before the surgery. Starting to make changes now will help you adjust post-surgery.
Make sure your pantry is stocked up with items suitable for each phase of the post-surgery diet, - liquids/protein shakes, purees, and soft foods. You don't need to get perishable items yet, but make a plan for what to have on hand to make those phases easier when you get to them.
If you drink a diet or regular soda frequently, start weaning off of it. If you drink coffee, convert to decaf ahead of time.
Make sure you have gotten all the information possible about your post-surgery diet and recommendations on exercise for the first weeks after surgery.