You can try using canned air around the outside of the wheel. You can also try on the inside around the 4-way cursor.

There is a spring-like piece of metal in the unit that exerts very minute pressure on the wheel. That is the source of some of the friction.

Aside from this, the only other recourse is disassembly. Which, if you do disassemble, I would recommend cleaning all the contact for all buttons with an engineering pencil eraser as over time, they do oxidize. Note that just cleaning with isopropyl alcohol will not work very well, hence the eraser. And while you are at it, clean the grime off the docking station board contacts. :-)