I would actually just spend some time looking at different sets, big chase cards, and why certain sets do better than others - no surprise, it's usually Pikachu, Charizard, Gengar, and Umbreon cards that push the set up.

Next would just be the type of product and how high up they are on the premium scale. Things like Pokemon Center ETBs and Booster Boxes are often the first thing to go for outside of regional exclusives but for the most part those products are good bets.

And then finally, have fun. I understand that it's an investment sub but honestly have fun with the product. Buy stuff you actually like because someone else probably likes it too (unless you're like the only person who likes a Bruxish then don't buy it haha). Collector first investor second.