What is a good way to remember which breast I fed from last?

Rubber bands. Put a rubber band on the wrist of the breast you are going to feed on next. That way when you begin the next feeding you can just look down and see which side to start on!
Use a safety pin . Use a safety pin as a marker. Simply place the safety pin on your shirt on the side that you start breastfeeding on. Next time you feed, simply switch the pin to the other side. If you wish you can make it more fancy and use a decorative pin jewelry or brooch.