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Well, the best deterrent is to be personally aware of when you have people attempting to break into your home. I suggest you do this by buying a rocking chair and shotgun. You can than sit outside your house cradling your shotgun, and glairing at the neighborhood kids. That'll prevent vandalism, for sure.

Kinda like this, except with a better chair:

We have reported things to the police. They are actually pretty friendly, and have tried to help, but there is only so much they can do with no real evidence and no witnesses. But they do patrol our neighborhood fairly frequently, just to make sure things are in order.

And to the shotgun idea: Definitely effective, but probably frowned upon. But I got a laugh out of it.

Have you thought about contacting the teens parents? Or setting up a neighbourhood watch?

I must say the sticker idea was a really good one.

I just read in the paper about a town that was plagued by teen vandalism.
They came up with a rather novil solution that seems to be working rather well. They started an order of Knighhood for the town. Kinda like scout and kinda like the SCA they appointed all the teens into it and would have round table meetings to discuss things like honor, virtue, the nobility of doing the right thing, with rewards and ranks for being good knights. Now they have a large cadre of teens who don't toleate holiganism and are very active themselves in keeping it in check.
It gave them a chance in a real life environment to see the value of these priciples and see the rewards that come with them. In addition they feel included in keeping thier town a cool place to live in.

I'd say to report the vandalism and they'll be on the lookout. Once the police start getting a lot of complaints they'll do something about it, or be more inclined to find out who is doing it. Other than that, tell people around you to be on the look out for teenagers doing stuff like that. Stay on the lookout yourself. If they come, turn on your sprinklers or something. Or hose them down.

hmmm....I suppose it depends on the teens involved but being confrontational could escalate and alot more than mailboxs and window could get damaged. Is there nothing in your locale to occupy these guys. I my home town to curb holiganizim on the weekends our local RCMP would hold a back country rally on saturdays (Scavanger hunt style not racing) and then the had a special deal on friday's for 1/2 off at the roller rink. Saved them alot of trouble and we appreciated the cops eek , these pratices stoped with a new police chief and now mishcief is a problem and the town now has actual street gangs.
The oldest addage sounds true for this sort of problem "Idle hands are the Devil's play ground", Talk to community members, city hall, inform them of your problem and connect it to idleness of the teens and get your community in gear to come up with ideas (they want your votes after all don't they) to occupy these youth. Otherwise that pic of the guy on his porch will be an inevitability.

Another idea that wouldn't hurt....

You said there is a neighborhood watch for your area? Well, see if you can't work in conjunction with them and possibly persuade them to go house-to-house (or you can do this yourself) and hand out a simple flyer detailing recent vandalisms in the area. It'd not only be a simple way to alert possibly ignorant neighbors, but also, if the vandals live within the neighborhood, their parents may get one of your flyers. The vandals would then see it and know at least that someone is taking some action against their malicious behavior. Maybe just knowing someone is being pro-active would scare them out of vandalizing.

Also, if you're not getting much help from the cops/other neighbors, then perhaps contacting your local councilman or the mayor wouldn't hurt. I know my Grandpa (the spunky old guy he is) will ring the mayor's phone off the hook until something gets done about whatever problem is going on. It's annoying and tedious, but sometimes persistance pays off.