Keep your hands as clean as possible. Try to avoid using the phone after anything dirty (eating, working outside, etc) Alcohol swabs or Q-Tips once or twice a month. Use a slip holster if you carry it in your pocket or purse. Dust under the trackball is your biggest enemy. A can of Compressed Air as part of your cleaning routine doesn't hurt.
Over time, its still going to accumulate. Human skin is constantly exfoliating, and combined with our natural skin oils it makes for a sticky gooey mess over time. Just ask your Car's steering wheel.
If I knew where I was going, I might already be there. -- Cross Canadian Ragweed.