I find it easy to base it off of something. Like if you're making a city, and working on a building - go for a walk, or look for specific types of buildings on Google. I sometimes use Google's Street View to get a good idea. Then just recreate the building as closely as possible, but with the twist that it's now made out of plastic bricks.
It's pretty fool-proof actually, there's no way you can go wrong. Unless whatever you're using as a reference does look like crap in real-life.
This can be used for anything really though. Parks, forests, interiors - anything really.
