I control my bag space by throwing Greats and Ultras when catching manually leaving the Red Balls for the Go Plus.
Yes there are plenty of occasions where the Red Ball count gets very low. When that happens I turn the get Pokemon function off and just let it flick stops and Gyms now now too.
I used to scrap lots of Potions and Berries. Feeding Pokemon in Gyms is great as anytime I need to reduce their numbers I just click on a Pokemon in a Gym and chuck a fruit basket at them collecting Star Dust and Gym XP at the same time.
I have a massive stock of Maxi Potions and Max Revives (over 500 combined) so I trash all the other potions and revives constantly. The inclusion of these in Raids has upset me as they are of no use.
I know where all the good ball run loops are in most areas so doing a few laps refills it quickly.
Another good tip for managing your bag space and being able to replenish it quickly is to not let any of the items get excessively low. Not sure if you've noticed when your out, or almost out of something the game doesn't give those items up much and it takes a while to build up while everything is building up quickly. The Wife had her Ultra balls get below 20 while mine were over 100. Several nights in row I went down to the local loop and walked it for 90mins at a time with mine and hers device. I was getting 3-4 Ultras to her 1 until I had over 50 on her account than the frequency spun up started climbing.
The game AI knows what you need and makes you play longer to get it.