Pull up random approach plates you have never seen before (large cities in other states for example) and practice briefing an approach. You will almost certainly run into something you are unfamiliar with and it will be a good chance to learn about it.
Always challenge yourself to learn new things, as well as keeping sharp on the things you have learned already. It's embarrassing when a student asks you a question and you can't remember how to explain it...
IMO he shouldn't force you to slip every time. He should be teaching you different techniques and not just the way HE prefers. There are often multiple ways to accomplish a task and (as long as it's safe and within standards) it should be your preference.
61.183 (i) states you must (1) have the endorsement and (2) demonstrate instructional proficiency... No time limit is stipulated... however, I would think if the endorsement is not recent (which would still satisfy the requirement) the examiner will want to see you have the proficiency