Hey there,
I think for me, I've learnt to rest. I used to have a really bad habit of going really over the top whenever I had a good day, I'd over work myself and then end up exhausted and back in a flare. The trick is to ease back into life, listen to your body and try to eat well if you can (I don't mean lots of veg, but lots of energy rich food). If its been a few weeks or so and you're still feeling really run down then its worth getting some blood tests done to see if you have any vitamin or mineral deficiencies that need addressing.
I have been told by my IBD team that fatigue is a major part of having a chronic illness, and that even when not in a flare, I should expect that I won't have the same energy as I used to or as the people around me, so learning how to pace myself has also been really important to me. I've stopped worrying about things that don't really need to be done, and I've learnt to prioritise the vital things first. This way anything that absolutely HAS to be done gets done, and then any reserve I have left can be used for other things or can be turned into something fun.