Keep up with your items. You have a limited number of item spaces and this fills up remarkably quickly if you’re hitting a lot of Pokestops. Want to use up some bits? Try dishing out those Razz Berries to hard to catch Pokemon or change your moves with Charged TMs or Fast TMs. If you get sick of spinning the icons on Pokestops and being told your bag is full, you can upgrade your item space by spending some very real money on Pokecoins.
You can also delete items in your pack by hitting the trash can and then selecting how many of those items you’d like to drop to give yourself more room for Pokeballs. Keep in mind that you probably don’t need as many regular potions when you have super and hyper potions, or revives when you have plenty of max revives, and you’ll keep on top of your space without much extra investment.