As Valygrl said, make a list and then organize it.

The planning I do is proportional to length of the trip. For a week trip, I'll have a more constrained route figured out but no need to turn off electric, etc. I've also found that I'll pack more if I do it too early - so typically wait until two nights before departure and then pack things from my checklist.

For trips over a few months, I'll have been anticipating them long in advance. I'll typically have a web site set up including links to other journals I've read on the area. I'll have done short shakedown trips with any new equipment. I'll have my own checklist of what to wrap up when, when to get visas/immunizations, etc. That length of time typically coincides with a job transition, so I'll have that wrapped up. I'll have found someone to house sit. I will also have gone through some contingency planning.

I'm also a little cautious about over-planning. Except for the shortest trips, I don't like to pre-establish my stops or lodging setups - at least beyond perhaps a hotel first night on arrival in a new country. I'll know an approximate route, but certainly not down to individual maps or turns. Even for shorter trips, I'll sometimes leave my endpoint with flexibility by booking two different car rentals (that I can cancel) or returning via train with knowledge that I should be able to adjust a ticket to do a subset. I might have done some research about various road choices, crossings of major rivers like Mississippi, which towns have accommodations or bike repair/resupply possibilities. However, I plan with a level of flexibility in mind.