This is called Batesian Mimicry in the wild. There is plenty of evidence to suggest that even if you don't get an alarm (if you can't afford it), getting the stickers and signs can deter crime as well. I am looking forward to what others say as far as what is best.
Looks like a lot of people recommending home rolling. I have seen cases of people who have their own home systems catching video of thieves stealing neighbor's cars and the like (without the hassle of going through the security company to get the video).

