Try using a variety of tiles to create more interesting flow patterns. Also be aware that you don't have to 'tunnel' the flow. Some of the neatest moments in races are created by dropping through a huge chasm filled with mines, or even jumping up through one (Though that's probably a poor example).
Check out these maps and maybe they can help: Flow Assists [nmaps.net]
_destiny^-
This is a good start
Check out these maps and maybe they can help: Flow Assists [nmaps.net]