A couple of years ago, someone on the forums (I think it was Fubar) suggested spraying the frame with silicon spray to help keep it clean.
By golly, to some extent, it does work. I don't like to spray solvent based liquids on the decals, so I just spray the silicon on that that difficult and congested area on top of the bottom bracket where the chain stays, post tube, and down tube come together.
It doesn't keep the area much cleaner. Mud and slush, and sand all still collects there, BUT it IS easier to clean off if you have previously sprayed it with silicon spray.
Be careful not to get the silicon spray on your rims or your brakes won't work.