try to cry when you are in private even if you don't feel like it like watch a really sad movie and cry until you can't cry more, to release any built up tensions then hopefully you won't feel like crying when you are in public.
Heres what I think are somewhat useful threads for you to browse.
http://ask.metafilter.com/78272/I%E2%80%99m-not-crying
-My-eyes-are-just-a-little-sweaty-today