A bird in the hand is worth two in the bush.
A stitch in time saves nine.
Don't let the cat out of the bag.
Don't open a can of worms.
He who rides a tiger can never dismount.
Don't swap horses in the middle of the stream.
Blood is thicker than water.
Let a sleeping dog lie.
If it ain't broke, don't fix it.
Ain't never been a horse than ain't never been rode, and ain't never been a rider that ain't been throwed. (While we question the grammar, the idea has its merits.)
Take a saying, any saying. Look at it carefully. Got it? Okay, now use it to launch a new story thought, either on an existing story that's stuck in the mud or in a brand new one just waiting for your unique perspective!
Here are some more:
People who live in glass houses don't throw stones.
It wasn't raining when Noah built the ark.
A cat may look at a queen.
No copyrights here. Write on! (And on, and on, and on...)