It's hard to go wrong with big franchise items, Star Wars, Superheroes, Harry Potter, etc. But sometimes the most valuable ones aren't famous characters, but obscure ones. It's a learning process like anything else, but after awhile you can tell what has potential and what is junk.
Even if you end up with less valuable stuff it can be used as parts in other sets at times. When I get those disappointing ones I see them as parts to add value to something else in the future.
!BBH
!LOL