Some people find crying onstage incredibly easy. Others (myself included), find it takes a bit of work. Here's a few tips.
As just stated above, trying to remember sadness from your own life can trigger the crying. However, often it takes a while to occur, i.e. the crying does not come immediately . Take a moment before you go onstage to really think about a situation that has/would make you cry.
There are other methods of, to put it bluntly, 'cheat' cry. While on stage, if you stare directly into the stage lights, it will often cause the release of tears in your eyes. This can be useful if the above method does not work.
Finally, if you cup your hands to your eyes before you cry, then this action could work for you. Before you go on, dip you hands into some water, wetting the ends of your finger tips (make sure it is not dripping). Then, before you need to have tears, you can rub this water into your eyes, giving the appearance of crying.
Hope one of these methods work.