Try something very different from what you have been brewing.
This helps me get through the humps....It could be falling in love with a new beer style, then coming up with my own recipe. That involves two of my favorite things, trying new beers, AND doing research, especially for historical recipes.
I'm suddenly getting into Kolshes after the Iron Brewer competition I participated in two weekends ago. Where my teammate and I decided that making a Honey Kolsh would be the best way to highlight the secret ingredient which was ground fenugreek (which we mixed with honey and some fresh ginger and caramalized in a dutch oven, like making a bochet.) Kolsh was never a style I drank much of, now I'm curious about them and picking up what scant few commercial brands I can.
SO that's got me interested again.
Look at a style you've never done, and push yourself to research and brew it.
Another thing is to do something hobby related but not brewing related, that would make you then want to test it by brewing, i.e. Make a new brewing gadget or tool. Do you batch sparge? Make a sparge arm and try fly sparging for one....Do you always mash in a cooler? Try BIAB for a batch, see how it works for your. Haven't extract brewed in awhile? Challenge youself to make a great extract with grains recipe using all those great tips you've learned since that first cooper's kit....Never done AG before? give it a try.
Never played with water chemistry try it now.....
Do you always brew kits? Try creating your first unique recipe yourself.
Just try to change something up and see if it sparks it.