Contrary to what some posters might say. I've never actually had writer's block... I'm just lazy.
My definition of writer's block would be; being stumped as to what to write even though I feel like writing (the physical act). To me, if you can reach a point of 'wanting to' sit down and write, once you start, stuff happens. The ease of the flow of what you write might be radically different from time to time, but words will appear on the screen (page).
Back when I was writing, my biggest problem was just wanting to indulge in the physical act of writing. The fun part, which is the creative process, is all done mentally. The actual writing is work!
As far as how to work out plot problems and tweak your ideas. I used to get a lot of 'internal brainstorming' done while driving to and from work. If I was in the middle of a story and wanting to figure where I needed to go next,(or as some have said, figure out the bridge needed to get to the juicy part I knew was coming) I'd turn off the radio and let my mind wander.
The funny thing is... Often while I'm at work I'd be all jazzed up to get home and do some writing. I'd have some ideas that I thought would be great, or whatever, but once I was home, the mood would pass.
Basically I used to have to force myself to sit down and start writing. Sometimes I would get on a roll and produce quite a nice chunk. Other times it would come much more slowly. The true 'block' was just getting myself to sit down in front of the screen.
Tank (who says that he is glad that he doesn't have to deal with those issues anymore)
Start writing the stupidest, most out-of-character stuff you can think of, and see if they don't come running back to set you straight on what they'd 