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.
Step 2: Reduce Visibility of Wear Marks
If there are wear marks in the black metal that you want to make less obvious, now is the time to go over them with a black permanent marker. Try to only color the marks. When done, use a gentle cleaning/rubbing to remove/blend any excess marker ink.
If there are unwanted deep nicks and engravings (such as scratched-in ID numbers), they can be colored by this method but not filled-in. If you want to try to fill-in the marks, wait until the next step and then try using several passes of black acrylic paint.
Step 3: Re-Color Engraved Markings
Although colored engraved lettering often is quite readable once cleaned, we are not always so lucky. If the engraving is still intact, I've found that unthinned artist's acrylic paint (which allows water clean-up when wet) can be applied easily and the excess rubbed off. A toothpick is one of the better applicators for fine engraving and a bit of paper towel or a cotton swab both work well for removing the excess.
This approach also works for mount alignment dots. In fact, the Sony NEX-5's E mount uses a white alignment dot, but 3rd-party E-mount adapters all seem to have red dots. Thus, I've been using this method to change red dots on brand-new E-mount adapters into white dots.
In general, the first coat will leave a very thin layer in the engraved area. Coverage is great, so one coat should be enough. Additional coats, with appropriate drying time between, can build-up paint to fill the indentation.
Step 4: Replace the Focus Grip
To replace the focus ring, simply measure and cut a strip of sticky-back craft foam sheet. Starting at the bottom of the lens, wrap the strip around the lens where the focus ring was, trim the strip to length, and press it down. If the ends pop up slightly, you can use a tiny bit of glue to help them stay in place. Craft foam isn't very durable, but it is cheap, easy to use (cuts with scissors), and has a great "cushy" feel.
The feel of craft foam is the main reason I prefer it to other possible coverings, most of which are more expensive but more permanent. Most leathers do not give enough grip. Sugru could be very effective, but would tend to give a very hand-made look.
Craft foam sheets come in black, but also come in many other colors -- like the orange used on the lens shown at the start of this instructable. Color-coding camera equipment can be a useful concept... or color can be a fashion statement. In fact, this material even comes in glitter sparkle colors, so you could probably color-coordinate your lenses with bridesmaid dresses.... ;-)