Like Kat said, it's the initial hurdle for me, too. The fears and demons (whether its practicing, writing, or whatever the thing is I'm avoiding) generally stay gone once I get started.
The best thing I ever did for my guitar playing was to buy a wall hanger and keep the thing on the wall in my bedroom instead of in its case downstairs. I play it for a few minutes more frequently PLUS I get in more frequent focused practice too. More frequent dusting is a small price to pay.
-Glenn