While pondering on my productivity, and the way I implement Getting Things Done, I've come to realize that there's a couple of classes of things I keep putting off if I can.
One is scary things. Things that, for whatever reason, make me afraid. For example, investigating a bug report because it might turn out to undermine the entire application and make it useless. Or looking at a new program someone else made because it might make my own obsolete. Or looking into switching to a new bank because it may reveal I've been mismanaging my money for years.
The other is hard things. Things that require me to think deep thoughts, or learn new things, or which might fail completely and make me look bad.
If I don't have an external motivator, such as a deadline from work, it's very easy for me to just do something easier instead. This is bad, because of then the scary or hard things are the most worthwhile to do next, since they give the biggest benefits.
Thus, when picking off items from my next actions list, I should pick the scariest thing first. If nothing scares me, then the hardest thing.