When you are working, that is output. When the well runs dry, that means you didn’t get enough input and/or you are burned out. So when you are not working, go out and hear live music, have interesting conversations over drinks with friends, read non-fiction books about all kinds of topics, and other things that will fill up the well again. Also spend some time doing nothing — meditating or daydreaming. Do things that use different muscles than your work. Getting away from screens can be especially helpful.
Another thing you can do is automate some of your work. Learn to script your main app (Photoshop, Illustrator, whatever.) If I have to create something in various formats, I’ll just make the one largest/highest-res format and then write a script that creates the other formats from that one master. It is not as simple as just resizing the whole document — you may have to hide some elements, resize some others dramatically, resize others not as much, and ultimately trim the canvas down to the new size. But the script you make essentially becomes an assistant who doesn’t know how to do creative work, but he knows how to take the poster you made and make a postcard, business card, and mailer out of it. A key thing is that then you can revise the poster, run the script again, and almost instantly get a revised postcard, business card, and mailer. And you can adapt your script for the next project and the next project.
Scripting doesn’t just save you a ton of time — it also saves you from the burnout that the grunt work of measuring and resizing again and again can impose on you. You end up spending 90% of your time doing extremely creative work and 10% of your time scripting, instead of 50% creative and 50% grunt work.