Begin by trying a bulb blower like a Giotto Rocket Blower. If that doesn't loosen it, you can try a wet kit (like a Copperhill kit) or a local camera shop will often do it. Barring that, you can send it to the company to be cleaned (Nikon in your case).
The problem is, I don't think the D60 has a manual cleaning mode, so I'm not sure how you're supposed to open the shutter and expose the sensor long enough to clean it.
The problem is, I don't think the D60 has a manual cleaning mode, so I'm not sure how you're supposed to open the shutter and expose the sensor long enough to clean it.