Why stopping them? Weeping is often necessary, it helps releasing - if you keep your tears in your heart, they'll become a sort of poison and you'll feel worse and worse.
However, the best way to stop tears is probably improving your mood. You may look up, as Ruby said, to the sky or to the future (also because then tears have some problem in falling); you may listen to a song you like and sing along, you may eat something sweet you like very much and have seldom (these two work a lot with me); you may talk with someone close who understands you; you may go for a walk...
:)