It could also be due to teething and even genetics, although Joey had a bad case when I switched foods, so allergies are something to consider. Here's a great link on causes and cures. Remember, what works for one person won't work for another; it depends on what's causing the problem. This time of year there's a lot of seasonal allergies; Joey's the only one in our family not having a problem right now.
Tear Staining: The Causes & Cures