Don't force your textures to fit your polygons. Your textures should always retain their original shape. Always work within the same texture set unless it looks really good. This is due to the way Quake2 lights textures and displays things. Not that the engine displays different texture sets differently, but they all have a theme or set of single-player levels that they attach too and using textures from the same sets eliminates one step of the process of putting it all together.
Work in 8 unit measurements. Try to make your ceilings 128 + 64 high. 128 units for the wall, and 64 for the trim on the top or bottom. If you have trim on the floor, make sure it's consistent throughout the level.
Don't start with a box connecting to another box. Start with a room and on how ways you could exit that room and leave yourself holes. When you're done editing that room, start on the next and thing of ways that room is different from the last and how you could connect it to another area.....and more to ocme later.
I don't start by making a room and thinking on how to connect it to others. I draw a complete layout (floor-plan) of the map with pencil and paper, thinking about nothing but gameplay.
Then I build an alpha version (no lighting, nodetails, ~3 textures, but with items placed) to test "travelling times" between items (MH->QUAD etc) and gameflow by playing it against a friend. I also check for LOS problems, jump distances and things like that.
I write down error I find and change small things. Then I think about the theme of the map and textures (often an idea has already evolved in the back of my head in this state). Then I build the map, using the alpha as a base.
I couldn't help but laugh when spirit said he creates an alpha version with no lighting and using 3 textures...that has been the theme of the last couple of maps I made for the Neal&Bob maps. Ok, I acutally used lighting in I/O, but it's really hard to tell, which was the look I intended anyway.
Ok, seriously...as far as layouts are concerned, I agree with the idea that you shouldn't start with a box and connect it to another box. Rubik's Cube's are boring...and mapping in this manner for a first person shooter is no different. Aside from that obvious fact, another thing to consider is that a box tends to limit the mapper's imagination and creativity. I can assure you that the majority of good/great mappers out there don't do this and the result is usually a sprawling level that has a good, unpredictable flow for gameplay.
I also agree with not stretching your textures unless you are intending to do something different, such as make a surface with properties similar to chrome (enlarged textures from stock size reflect more light making it seem like shiny metal). Keeping everything within 64 unit increments also makes things easier on the engine when compiling and lowers r_speeds during gameplay (it also makes it easy to apply the textures since most textures are in measurements of 32, 64, 128 units).
Ok, so this thread is really old, the information is still good. Hope it helps!
If a person lives in a bathroom, would that make him/her a "loo tenant"? - Philosoraptor