My method is:

Spray the rim down with WD-40. Make sure to get the nipples as well. Take some scotchbrite and spend a few minutes scouring the brake tracks. Then spray down the whole thing with soapy water or cleaning solution and wipe down. Drop a bit of lube in each nipple and wipe down again.

This gets the brake tracks looking like new and lubricates the nipples so that the wheel can be trued easily.