What motivates you to write? Some would say the need for money. But, you could do something else and make more money, likely easier too. Money is a motivator, sure. But, what keeps you from giving up? What gets you typing on your keyboard (or picking up a pen) even on days you could be doing something else, like procrastinating?
Knowing your motivation makes it a lot easier to find on the days you need it.
Source: Some Motivation Required Shark Remix – Shirt.Woot
The shark is my favourite. All the designs were sold out when I found the site. But, they are likely to have more when sales are going well.