OK, first thing is to be very gentle! You don't need to apply pressure to get these techniques to work. Don't do anything if the picture appears very dry and flaky - you might lose the surface altogether.
1. Using a very soft brush (like a blusher-brush or a very soft chinese calligraphy brush), start in the middle of the picture and brush gently outwards using short strokes. This will get off any loose surface dust etc.
2. Take a cotton wool ball and using the same technique, move it over the photo. If it's picking up any dirt etc. you will be able to see it on the cotton wool.
3. Rinse your mouth out very well using water and a toothbrush (but no toothpaste). Take a cotton bud/Q-tip and stick one end in your mouth to get a bit of spit on it. Start at an outer corner of the photo - hold the cotton bud at as low an angle as you can (i.e. as flat to the photo as possible) and roll it back and forth across the photo. As the cotton becomes dirty, start with a new one.
Hopefully this should help it come up a bit cleaner! If you do try any or all of these techniques, let me know if you have any success.
Cheers
Prue
