If your are flying in creative and you fly up and then hold down sneek, you will stop flying. (intentional)
Then if you let go of the jump button, intead of descending you will remain standing.
THEN, if you let go of sneek you will start continuously fly upwards (without pressing anything) untill you pause or open up your inventory.

1 Fly up
2 shift
3 let go of fly
4 let go of shift
5 you will be flying upwards without doing anything.

(may take one or 2 tries)