I really love using old lenses on modern digital cameras, but many old lenses have cosmetic issues that make them a little less pleasant to use. Here are a few very cheap and easy things you can do to make these old lenses a little nicer to look at and to use. I don't advocate doing this to rare collectible lenses; this is for "user" lenses.
Note that these things have nothing to do with internal functionality of the focus or aperture, nor the condition of the glass. That should all be good before even thinking about this. No sense making lens ergonomics better if the lens isn't known to be worth using!
Step 1: Prepare The Lens
Do whatever is needed to clean the glass and ensure the innards are operational. If the lens has loose parts, tighten them. If your focus ring is going to be replaced, remove it.
Now clean the lens body. Wet cleaning works best, but remember that "wet" means ever so slightly damp. No liquid should be able to get inside the lens. Soapy water, alcohol, and very sparing use of other solvents can all work on metal-bodied lenses, using either paper towels or cotton swabs. A little alcohol on a cotton swab is usually best for cleaning details like engraved lettering.