Hi Cat
I've written several 10 minute plays and a few that ran close to 15 minutes. Tell me if I'm wrong, but I think when you say a short time constraint, you're referring to the actual length of the play? As far as my experience goes, it was initially difficult because I believed that - like full-length plays - even short plays had to have a beginning, a middle and an end, along with a plausible plot line, conflict and great characters. However, I think that with such a short play, you just need to concentrate on one aspect of these factors that make up a play. For example, put some really strongly drawn characters into a funny situation. Or take a classic kind of scenario and use your characters to twist it around. My most recent short play was three men in a bar disagreeing about fairy tales and the effect they have on kids. No themes or messages in there; just funny.
Hope this helps,
Mandy