Usually I just start off with a sketch of how I want the map to flow. I mean, like big squares representing areas.
Then I look at those and see how can you make routes in-between them.
Then, I try to make those routes as interesting as possible in terms of what they are (i.e. an elevated, yet exposed pathway going alongside the main route.)
From there I keep building an area from that, such as working on various heights and features until I think that an area would be interesting.
As for expanding upon an area, if you've made exits out of an already blocked-out area, you can try just expanding them naturally. Obviously, if you have a staircase as one exit, it simply won't do to have it end evenly with that tunnel you made. In that case, you already have some nice vertical play in the next area: one higher, coming from the stairs, and one lower, coming from the tunnel.
Another good tip of mine is that if you have an already blocked out area in Hammer, trace the basic layout (in 3D) of it onto a piece of paper. Then sketch out ideas from it. This can also help with theming: If you've got a tall building in one area, try extending into where the other area would be, etc. etc.
Mainly though, it helps to start with a full sketch out of what the map will be. This will help in planning and in execution.