Yes, if you flare while on azathioprine, you have to take steroids concurrently with the aza.
If you have too many flares on azathioprine, your doctor might consider switching you over to 6MP or methotrexate. Another option is to just move to biologics.
Your doctor will usually ensure that the level of azathioprine in your body is correct for your weight. It doesn't work in the sense that you increase the dosage of azathioprine during a flare.
Personally for me, I still live as per how I did before I had this condition. Probably just more conscious of the foods I eat and taking care of my body (sleep, stress levels etc)
Hope it helps and best of luck with your condition!