That's one of the big issues with the game, especially if you're kind of plunging through it. I tend to not have any trouble staying on top of things, even with a full team early on, since I do a lot of back tracking. I like to do as much as I can in the game, so I end up going back for Bug Catching Contests, hunting down Pokemon for the Pokedex, and training other Pokemon that aren't on my permanent team. Also registering some later game trainers for rematches will help.
But yeah. The level curve in GSC is really bad. ;(