Since I’ve done so many maps I’ve basically broken mapping down into distinct phases that I personally follow.  I can usually get maps done rather quickly with this method it seems like.

1. Draw it! Get your design down on paper in a rough layout.  It’ll most likely change later.
2. Create the blocks / Cannibalize! You can create the bones of each section within a rather short time if you have a clear goal.  Cannibalize parts of old maps you may have such as spawn doors, and be sure to use existing door frame models to help scaling.  Use dev textures such as the red/gray.  Stop at the exact middle of the map so when you mirror it with later it fits well.
3. Test! Run through your map and show the layout to a friend or two who knows what they’re looking at.
4. Texture! Put textures on those buildings and make your terrain (most of it).
5. Basic Lighting! Make it so you can compile lights and start getting them looking nice early.  You can cannibalize some fixtures from your previous maps to start with.
6. Details! Make those roofs and start putting in brush details and key models.  Don’t forget overlays and all the cool props.  Open a Valve map to check out good props to use.
7. Clipping! Make sure players can’t go where you don’t want them and smooth off rough edges.
8. Optimize! Put in area portals and hint brushes.
9. Clone your base! Copy the side you’ve been working on and rotate it 180.  Copy it to a new map file and replace the colors in a text editor and in Hammer.  Copy it back after and line it up with your other base.  Make tweaks to joining details as needed.

These are the basic steps.  Some may choose to clone their base earlier to test gameplay, but I prefer a pretty map before the serious testing goes on because people give them more attention.  It may be more work in the end, but I don’t mind.