You might also try to write a series of flashes with the same characters and setting. Maybe they could then be melded to form a longer work? Just a thought.
Plot complications are good. So are characters you know in depth, and who have extra backstory. A non-standard setting, if that's where your affinities lie, can also be pretty good.
I've stopped flashing in fact, for the time being, just to try and break out of the rut.
My solution so far, and this is the first one I've found that seems to work for me, your mileage may vary, is to write down every single specific scene idea I have for my novel. Then go back and figure out an approximate word count based on how much I need to cover in each scene. Once I have a target to shoot for, when I'm writing I can look at my projected word count and try to make the goal, while not padding my prose unduly. Just taking the time to flesh things out a little; putting in bits of description that I would have cut in flash, that kind of thing. Seems to be doing well so far.
For example, I wrote out an outline of each scene that was fairly detailed. I estimated my outline would be 4k. It ended up being 5k, which is fine by me. Longer is okay at this point. If I need to cut bits of the novel later, I'll be able to. I'm just getting the dang thing on paper.
Flash fiction versus short stories versus novels... All fiction, all related, but they require different skills.
Try plotting the story out, adding more plot details. Put your characters into the worst positions you can think of, and make their lives complicated and messy.
Good luck.
So I broke every sentence out into its own paragraph, and then wrote several more sentences to finish each new paragraph. The end result was about 3 times longer than the original, and I'd found ways to work in a lot of the information and nuance that I needed to. It's also repetitive and dull in a lot of places but for me it is much easier to cut than to add, so that will be easy enough to tidy up.
Now if I wanted to take that thousand-word piece and expand it into a novel, I'd need to add several more fairly significant characters. I'd make sure they're all in conflict with each other, and that each has problems of his/her own that need to be resolved, preferably in ways that conflict with other characters resolving their problems. And then at the end I would have the evil robot monkeys kill them all.
I guess that you could begin with a short flash as Beth has done, but instead of focusing on every sentence, focus on particular areas that you know could be expanded, like details(Sights, sounds, smells,tangibles) and characters.