Finally getting diagnosed has helped me immensely, and reading books or watching videos or listening to podcasts by other autistic people has as well. It helps me learn new ways of coping that I might not have discovered on my own, and helps me to feel less alone.
Beyond that, I have found that paying close attention and listening to my body and mind when it is telling me I need to rest is essential; I used to feel guilty about taking that sort of time for myself, but I don't any longer. I believe the role of fatigue in autism is under-emphasized.
Losing myself in one of my special interests helps, because I forget about everything else.
Other things: Solitude, silence, wearing an eye-mask to shut out the light for a time while I rest, listening to binaural music on headphones, allowing myself to stim freely, learning to say no to things I don't want to do and generally setting healthy boundaries with other people, creating a cozy and restful space that suits me (for me it means a quiet room filled with books, a comfortable chair for reading, a comfortable bed for resting, and I have several different lamps in my room so that I can always adjust the light to suit my mood). If there is a lot of noise in the house or outside, I have earplugs and headphones that I can wear.
Walking or other exercise helps me a lot.
If I am just generally uneasy and restless and can't settle to doing anything in particular(which does sometimes happen) I find that doing some small chores or organizing something can help, and I have a feeling of accomplishment after doing it that feels good. It might be something as simple as straightening the books on a few shelves, or bigger, like completely rearranging a whole room, which is something I actually really enjoy doing.
Journaling, just writing freely and doing a mind-dump; getting things out of my head and onto the page is something I've always done, and have always found helpful.