One of the things I've found that truly helps me in working on a campaign is creating monsters or factions. Worldbuilding through tangible statistical creatures seems to really help me. For instance, instead of just picking through monsters and placing them, I'll figure out where a creature would be, and then name it something along the lines of Ruby Goblins of Rime (which you can see on this blog, just go to the Monster page). Since I want to run BECMI, I have to re-stat them a bit, of course.
The process works for established monsters as well, if I give them an ability or a stylistic tweak or a personality change and make them fit into a region, such as a group of dwarves (and I completely stole this idea from somewhere but I can't remember where) that have gone insane and are trying to build a stairway to the sun so they can mine it, in the belief that it holds unimaginable riches in ores and gems to make it so bright and shiny. Imagine how precarious that encounter could be if a PC spouts off and begins to laugh at the idea that they take so dearly and seriously, with a bunch of ill-tempered dwarves now driven to insane anger. Those guys are going to go near to the equator since the sun is always high in the sky for hours on end, and led to their insane quest to try to mine it.
Anyhow, on the mapping front I am now 31 pages deep into the 49 pages of re-mapping the world it took to break things down into 6 mile hexes. It's a long, tedious process, and I'm trying to get through between 2 and 4 pages per day, so I'm very close. For individual adventuring areas I think I'll break them down further into 1 mile hexes and use a few resources like random tables to give me extra ideas and to tweak the terrain when it gets that granular. By the end of this, after I dabble in a few different beginning campaign areas, I'll be able to go on the hunt to start a game somewhere.