Noise is the best deterent alarm on the garage and chain the bike hopefully the noise will make them run off before they have chance to have a go at the bike. If they can get to the bike without making any noise chains wont stop them
Working in the security industry there are a few things to consider:
1. Make them go elsewhere, use dummy bell box', Dummy Cameras if you don't have the real things.
2. An Intruder alarm is only effective if responded to, so have a signalling system fit ( ADT mentioned earlier) without a certified system and URN no Police attendance, you will rely on a neighbour calling the fuzz. As a minimum fit a dialler to your system to call someone local.
3. Fit extremely loud Sounders to react on Entry. Master Blasters are 127db
4. Make it as difficult as possible to remove objects, chains and ground anchors- this is all aboout extending the time the shits have to risk being caught.
And my personal favourite, fit a Smoke Cloak, Strobe and Sounders, if you haven't experienced a cloaking system you cannot see your hand in front of your face, with any luck the bike will fall on the twats! http://www.smokecloak.co.uk/en/