What methods and techniques have you found to notably reduce your completion times in speedrunning, and how do these differ among various games?

It sounds odd but actually practicing choke points and specific strats in runs rather than doing a lot of runs is absolutely crucial. Even in games where I feel like I know exactly what I'm doing and what to do, practicing exploits and setups a lot makes it almost subconscious in runs.

Usually if I want to get a decent time on the fullgame boards, I'll get a feel for the levels first, or I'll just practice the entire game or marathon until I get world record or a good spot.

i lower my times by doing more runs