This is the hardest thing for me too. That's why, usually, if I don't know the end, I don't even start anymore.
I agree with what everyone else said. Awhile back, an editor told me that the end of a pb ms. wasn't working, that it was flat. Finally (a year later) I found a way to change the beginning, wrote another end, and now she's saying it works.
It also helps to read other people's picture books. Sometimes that will jiggle an idea loose.
The only thing I'd add is to try to reverse your reader's expectations. There's a pb (don't remember the title) that features a little girl preparing for a reading lesson with her grandmother, and the reader is set up to expect that the grandmother is teaching the little girl to read. But at the end, we find out that, no, it's the little girl who's teaching the grandmother to read. Very nice story.