Years ago, I worked with vacuum equipment, and one of the shortcuts taken by some of the equipment techs was to slather the o-rings with "guinea grease" if they couldn't get it to seal. I quickly learned that this was a pretty poor way to do it, since the equipment would leak again in short order when the grease got thin.
Granted, the vacuum levels of that equipment was a lot higher in magnitude than the pressure encountered in a keg, but I still feel that using grease to seal a o-ring is a bad idea. Sure, a little lube is a good thing, but don't be tempted to slather on too much to hide a bigger problem.
That said, the best course of action to me would be to find exactly where it is leaking, and attack the source. For the lid, definitely try re-seating. Ensure that the little grey caps are present under the ends of the clamp (if not, use a dime under it temporarily until you can get more). Try placing it the other way (180 degrees); that sometimes helps.
If the post is leaking while the gas is attached, it is more than likely due to the post o-ring. Try replacing it; if that doesn't work, the post may be bad. If it's the poppet, then try clearing it. I find that they sometimes gather a bit of gunk that prevents them from sealing completely.
Good luck.
