I do everything from a first-person PoV. Not by choice... it just... happens...
When I get stuck, or when I hit a point where the situation, scenery, or other characters feel wobbly or two dimensional... or when I just don't know how to get to the next milestone, I sit back and do a short story.
Here are the rules I use:
1) The story must be from another character's PoV. A supporting character, an antagonist, etc. It can also be from a "historical event" that affects the current situation.
2) If the short takes place in the same time-frame as the WiP, then the narratives can only brush up against each other briefly. I don't want to just tell the same scene from another point of view.
3) The story MUST work as a stand-alone. Knowledge of the other works might enrich the experience, elements might dovetail together... but it has to be done in a way that a person who's read nothing else in the storyverse can enjoy it without having read any other damn thing I've written.
This approach has helped me figure out where to go, solidify wobbly scenery, and flesh out supporting cast members. It's also generated full-blown characters and events that the WiP "needed" without me knowing that element was missing.
Oh, and of course More Stories is another happy result.