[Thomas]

For a more systematic, formalized (yet without a lot of must-do rules) approach look into GTD by David Allen. It includes a lot of the ideas discussed in this thread, including a stock taking at the beginning (the general direction is to create zero overwhelm and a mind without distraction by open loops).

A very short summary of what I think would be the best approach to prioritizing is to make sure you have clear purpose, values and goals. For finding your purpose quickly, Steve has written a very good article. As for values, there's another one, which takes a bit longer though (especially if you create an ordered list). As for goals, they derive from your purpose.

Obviously it's not as simple as saying my purpose is A, my goals therefore are x, y and z, and then focus only on those goals. Shutting out goals that aren't directly derived from your purpose isn't always and everywhere a viable (or good) option. However, the more closely your goals are aligned with your purpose (and values as well) the easier you'll be able to work on them, because the more passionate you'll be about them.