Writing to length targets/guidelines–how do you do it?

I'm trying to work on a piece that's supposed to be about 5000 words. At 3500 words it's not half finished, so clearly it will go over the length limit. It's not a disaster for this piece, but I still can't quite figure out how to keep my story line pared back to know that when it's finished it will be a particular length.

Is there a trick method for this? How does everyone else do it?

I usually come up with a story idea for a particular theme or topic, based on a call for subs. Then I sort of hope that I can finish within the guidelines while doing justice to the story. So far, I've failed trying to use only 30k on what should have been 35-40k story just to hit a target for a particular editor. It didn't work and I need to go back and flesh it back out before subbing again.

In general, should I outline it first, and then figure an average scene length to come up with approx finished word count, then leave out scene(s) that won't fit?

I have a few shorter pieces I need to write and I'm worried my ideas are just too complicated to fit into the length requirements and I need to either come up with simpler ideas or completely new ideas. Any help here is greatly appreciated!