A lot of people just learn the idiosyncrasies of each keg in their fleet, and treat it accordingly. I went the opposite route and tinkered with each of my kegs until they all sealed well under any condition. I've found that it boils down to three issues.
1) The o-ring needs to be in good condition, and lube can also help.
2) Making sure there aren't any dents, warps, or nicks in the metal of either the keg or the lid where they mate with the o-ring is important. Not much you can do about deep scratches and nicks, but wet sanding with very fine sandpaper can take the sharp edges off of smaller scratches. Warping or dents can often be remedied with a mallet and/or channel lock pliers and a piece of thick leather to protect the metal.
3) Many of the older kegs have significant splay in the bail feet, and using some vice grips to bend them back so that they exert more upward force on the lid can help a lot. Same result as the coin under the foot trick, but permanent and doesn't require fiddling with coins every time you open or close the lid.