For some coding task, I gently start thinking about [problemX] well in advance, and then forget about it and do other stuff. I don't even think a whole lot about the problem, just note that I have to do such and such, and maybe think a little bit about how I might do it, then forget about it. If it's an important/hard/interesting problem, the rest comes naturally... I go off doing whatever other piddling crap needs to be done, and by the time I'm ready to sit down to attack the problem in question, I usually start to get ideas, seemingly out of nowhere. I've been following this process for some time now. When ideas on how to implement something start to come, that's the zone right there, far as I'm concerned. I can spend a couple hours without looking up from the screen at that stage.

Long story short: for a problem which I think is going to be hard to implement/fix, I briefly think about the problem well in advance of sitting down to work on it.