This might sound funny, but I had a special potty-time sing-song count to five; they had to stay on the toilet until I finished counting or they peed, no exceptions.
My "song" sounded kind of like Dory the angelfish speaking "whale" in the movie "Finding Nemo":
wha-unnnn, tooooooo, three-eeeeeeee, fohhhhrrrrr, fihhhhhhhhhvvvvvvvf
(with "five" occasionally held out especially long if I felt the kid was holding out ;) ).
Singing the alphabet or any other singsongy thing they like that lasts a while also keeps them there, but let's them know you won't keep them there indefinitely (the latter expectation would be likely to lead to control fights).
Sometimes also I would end the song with, "Now: shhhhhhhhhh" (the pee sound) and kind of direct my attention/energy, and therefore their attention/energy, toward their urethra, to REALLY encourage them to let go (my kids tend to be the "I need to go but now I don't want to" types). Sometimes I pointed, or even poked their bladder.
(Being a mom is so crazy sometimes.)
Man, the going-on-the-floor phase seems sooooo eternal, every time ... it really isn't, but it SEEMS so. May God give you patience and perseverance :).