...a Big Fat Zero.
No, really. I started doing this for my workouts. I hate to workout, but know I need to. So, in an effort to get myself to do it more often, I took a notebook and drew out three columns on the page. In the first I put the date, in the next I put my weight, and in the last I put how many minutes I work out for the day. I soon found that having to draw a big 'ol round zero in that column was something I didn't like to do. I also don't like looking back at the page and seeing too many zeros in a row or even scattered about. I'm still not working out as much as I should, but I'm working out more often, just to keep from having to add another zero. (Smile)
So, why can't the same idea work for your writing? No reason that I can think of. If you have been having a hard time writing and just skipping too many days, grab a notebook—doesn't have to be fancy, just what ever's around—and draw lines down the page to make three columns. Put the date in the first, what kind of writing in the second, and the amount in the last.
Date, I don't need to explain. What kind of writing, well, you can just put the word Type, and then when you fill in that column use words like rewriting, editing, new writing, ect... Make that middle column the biggest so you have a little room if you like. You can even add things like promo, research, or such, depending on what you did that day. The last column is for the amount. You can put hours if it was research or promo or even a workshop you did on writing for income or promo, and for the actual writing you can put a word count, the hours, or the pages. Since it's your record, you can set it up to cover what and how you want.
The hope is though, if you start noticing days in a row where there is only a line of zeros, you'll try to make them appear less often, which means your writing will increase. (Smile)