For gameplay, I try to immerse myself in the map, I make a crude room in Hammer, fullsize the 3dview, and place myself in different areas of the room, thinking "Hmm, what if some guy came from over there, and then the aliens came crashing in from over there" and so on, which gives me a good idea of the layout and chokepoints of the map.
When I have the layout complete, I work on timing, thats to say, it should take an equal amount of time for Team A and Team B to reach the chokepoints and important areas, or if its Deathmatch, each spawn should be equal, ie. ammo and weapons nearby, not too close to superweapons and the spawn shouldnt be in the critical path of the map.
For design, I just get some cool ideas in my head, or I take a look at different texture sets and imagine cool setpieces around the textures, and then it just automatically gets more and more advanced.
If that doesnt work, I play other games or look at screenshots from other maps/games, and "steal" some ideas. It's not really stealing, since I never make a copy of what I see, over time the inspiration I got from the other game/map is mutated into something of my own.
Wow this was way too long.