First of all, don't beat yourself up about it, it'll only add more pressure, try and accept that you cannot possibly be prolific 100% of the time.
1. Take a walk and get some fresh air. Even if this is just a trip to the local shop to buy a cheeky chocolate bar for yourself (you totally deserve it)
2. Talk to someone who understands, a fellow creative. This can be online or face to face over some coffee. A little vent goes a long way, and it's also nice to perhaps get a fresh perspective.
3. Spend some time in the kitchen, no NOT cleaning, baking or cooking something yummy! I really like how absorbing it is and it gives you a chance to put your mind to something else, eat your new delicious treat and then come back to your work with fresh eyes.
4. YOGA! Yoga is my best bud, not only does it go some way to try and fix my nasty posture problems I have from hunching over my desk all day, but it allows your body and mind some breathing space. I can go into a yoga session completely stressed out and by the time I come out I can feel a lightness in myself again.
5. Work on something else, something that isn't for sale, that no-one else needs to see. Crack open a bottle of ink or paint and try working with that, push yourself and have a little fun. Remind yourself what you love doing and how awesome you are <3