Do pro-bono work
There are several benefits to doing pro-bono work once or twice a year. First, it's usually a nice ego boost for designers. Sometimes paying clients can be harsh or difficult to deal with, and (usually!) the non-paying clients allow you greater creative freedom and will leave you with a smile. I've also found they're very likely to recommend you to other (paying!) clients.
Read more about this at Lifehacker and at Fuel Your Creativity.
Find the right balance of background noise
While constant interruption and loud noises can be distracting, no noise can be very, very boring - and squelch your creativity. Finding the right balance of background noise will help your imagination to wander in the right direction.
I've found that I'm most productive in a coffee shop (surprise, surprise!), not only for the background noise and comforting beverages, but also for the atmosphere. Something about the quirky yet comfy surroundings makes me want to create.
For more, head over to 99u for more about ambient noise.
Keep a sketchbook
This one is obvious, but this is usually one of the first things to take a back seat to other priorities. I've found that if I pull out the Moleskine and sketch for just 10-20 minutes, it gives me fresh eyes more times than not.
I've only recently started keeping a sketchbook again, and I'm amazed how quickly I gain a spark of inspiration by just putting pen to paper. There's something very therapeutic about it.
Here's a great list of ways you could benefit from something as simple as keeping a sketchbook.