Do you plan out your Portal maps?
Ehh...
I have a small notebook I always bring around.
Whenever something comes to my head, I try it or write about it.
And now I'm pretty much using it to plan my portal map.
All I do is draw square rooms, then add all the stuff I can think of then yeah.
Then I check it over, see if there is anything to add, then add.
If I want the room to be curved, I take an eraser and erase some walls and stuff, etc.
For me that's different all the time. I created the first chamber of my current Portal map by just launching Hammer and trying a few things with brushes. Now the first chamber is as good as finished, I found myself drawing it on paper and trying to add a second chamber somewhere, that would fit the first one (in difficulty, setting and such).
Completely different is the TF2-map I'm also working on, I first made a detailed drawing of that before I did anything, and that appeared to work out great. Got most of the map in rough shapes now, gonna let the details up to my brother (he's great at that).
"Duct Tape is the answer."
Top down drawing
Orange map
Test
Refine orange map
Texture
Add lights
...
Orange map
Test
Refine orange map
Texture
Add lights
...
I admire your discipline
If I've got some free time I'll draw out a few squares and try and work some top down drawings, my lack of pencil skills usually just ends up in wasted paper though 
I've bought a large pickable whiteboard specially for Portal mapping, although I mostly use it for setting up entities algorithms, I use it somethimes to compose levels.
Author of Minecraft mods (MAtmos, Minaptics, NoteSlider) and Garry's Mod addons (Gunstrumental, SharpeYe, GarryWare, DepthHUD).