My rabbit has chronic pasturella. She lives with my other rabbit who was symptomatic with it for a very short time after bonding them, but after one round of antibiotics no longer displays any symptoms.
The one with the chronic infection has been on many different antibiotics. The antibiotics never get rid of symptoms, however I have found a combination that decreases her symptoms to being almost non existent, and extend the time between her "flare ups" or when her symptoms are bad. Macy's only symptoms are sneezing and nasal discharge.
For every rabbit this can be different, and I have found what works through trial and error and over long period of time, and with no lack of expense.
My vet and I have found for Macy's pasturella it works best to treat with antibiotics symptomatically, but to treat with other means all the time. Also this helps prevent the antibiotics from becoming ineffective which can happen if overused.
Each day Macy gets probiotics (I use benebac powder) and I also give her a multivitamin. I also have a humidifier in the room with her, and that has seemed to help. She gets a balanced diet including veggies, good quality pellet, and different sorts of fresh hay. I avoid using any sort of dusty things near her cage, as she seems to be more sensitive to dust things so I make sure my hay has had any dust shaken out, use a dust free litter, etc.
By using day to day things that boost her immune system, I have gotten Macy down to needing antibiotics about 2-3 times per year. Before doing this she was constantly on and off them every few weeks-months and having sneezing fits. When she would get bad she would sneezing around 20-30 times in a row. Now, most of the time it's one or two sneezes, but when she is bad and it's time for antibiotics she sneezes 5-10 times.
Macy is very responsive to baytril. We have started combining the baytril with nebulizing antibiotics. The antibiotic that I nebulize is gentamicin and I combine it with saline. Nebulizing uses a machine that essentially makes the medication a mist that the rabbit inhales. Combined, the two antibiotics work very well for Macy.
Macy did have a culture and sensitivity test done, which is a good idea because it tells your vet with antibiotics the bacteria is sensitive to essentially giving them a better idea of which antibiotic treatment it will respond best to.
Perhaps the hugest thing you can do is find a knowledgable vet who you feel like you can talk to. I've taken Macy to 4 vets before finding Dr. Heidi, and it was worth the wait to find a vet who is so willing to keep brainstorming new ideas and pushing to get my bunny in the best shape she can be in. She is also one of the cheapest vets I've found and gives me discounts for continued treatment. So don't settle for a vet who isn't willing to continue working with you to find the best solutions.