It's called "weeping". You need to reduce the sauce further. Also, if you leave large pieces of vegetables in the sauce (including tomatoes) they will continue to breakdown and release their moisture as well. To get around retained water and such, I use crushed tomatoes. Either buy them already crushed, or run them through a food mill. Don't blend though, as this aerates the sauce and suspends fibers which creates an orange colored gritty sauce.
A quick fix that is temporary for service within an hour or so is to make a slurry with Corn Starch or Arrowroot. To me this is cheating though, and it doesn't hold forever. Remove some of the water from the tomato sauce (make sure it's cool) and blend a good amount of cornstarch into it (in a real pickle you could just use some cool tap water). Allow the sauce to return to a good simmer (bordering on boiling). Add slurry in increments until it thickens up. This is really an emergency fix though, and not something I'd plan on doing ahead of time.
Nick ~ "Egg whites are good for a lot of things; lemon meringue pie, angel food cake, and clogging up radiators." - MacGyver