Any tips.... well there are some but the problem is, is that you get one shot at some of these. So you have to work quick and have a camera handy.
A few things people have had success with ... but of course first you need to know where to look 
1- Some have found that a coat of oven cleaner will strip the paint (while preparing to restore a part) but it can make a mark stand out more and bring the color out of what remains as the spot expands and dissolves. This seems to work where the mark was over a painted surface
2- Where the mark was over bare metal media blasting the area seems to produce some good results.
3- Where you have a collection (read drip or run) of paint from a mark. Always cut through it or sand the outer surface to remove the results of the elements and wear to see the original color (wet is s little with a little oil helps also)
4- With some be very careful with cleaners because some can be removed (crayon and ink marks) by simply wiping them with a cotton cloth.
5- Finally take pictures and look at them on the computer. I;m finding that for what ever reason I can sometimes see in the pictures marks that were not visible in person. Rare but I have one last wee this way. Really surprised me.... and I need to return to see how dramtic the difference may be
Hope this helps