Consult the wiki
Years of unreal experience dating all the way back from UT99. Seek answers in the wiki. Even the out of date stuff is still useless with some creative thinking and common sense.
Save, Save, Save, SAVE.
Save versions, make backups, save everytime you press rebuild. Come up with a naming structure, (RO-mymap01, RO-mymap02). Everytime you do something major (new terrain, new buildings, lighting, pathing, scripting) save a new version. If you change something and it doesn't work, you can always go back to an old version. UnrealEd Hates doing anything except completely destroying your map. Save save save savity save.
STAY ON THE GRID.
Static meshes are designed to use the grid, BSP will explode into a mess of crap if you start going off the grid. It makes your level run like grabage, look like grabage and turn into garbage
. I would recommend you do map planning at 1024, Large buildings at 256, interior planning at 128, fine turning at 64, details at 32 and stairs, trims and fansy stuff at 16. I would only ever go to size 8 to finely place a static mesh. Anything lower then that (4, 2 and 1) are just there for reference of distance and to tempt newbies to use then and suffer horrible headaches.