it's very difficult (read: impossible) to decide when and where to be creative or inspired. sitting in front of your computer with hammer open and forcing yourself to map is often an excercise in frustration, and can lead to self doubt or a lack of interest in mapping.
a lot of great writers carry a notebook with them all of the time, so that at any time if they see something inspiring or have an idea spring to mind, they can jot it down immediately. I've started doing that with mapping too, and it's helped immensely in actually being productive when i have hammer open. if im out running errands and i see a building with a particular feature, or walk through a room and notice something that i hadnt noticed before, its often enough to jog my creativity. i can't even begin to count the number of ideas that i've had that have just been lost to the ether because i forgot about them between the time i had them and got a chance to make them into a map. being able to write that idea down immediately or sketch out a 45 second layout to solidify that idea in my mind means that when i sit down to start mapping, i have a plan already, which is unbelievably helpful in cranking out layouts.
it's also important to recognize that creating a layout gets easier with time. with each map you make, you get more comfortable with the tools, you get more comfortable scaling appropriately, you get more comfortable with what kinds of features make for an interesting map. thats why one of the most important steps that you can take to improve yourself as a mapper is to actually put out a map, get people playing on it and testing it, and revise based on that. the more often you manage to get multiple versions of a map out, the better off you will be when you next start from scratch.