The bottom line is, If they want it, they'll have it.
Your job is to make it hard work, and to make them think..'Ahh feck it ... there's a bloke in the next street along that's got a bike in his garage ... lets nick that one instead'.
So to that end, try not to advertise the fact that you've got a bike (try and avoid the sunday ritual of 'bike on driveway with a bucket of soapy suds' for all and sunder to see - get it round the back and clean it ..),
always lock it (I agree with Trixie), maybe put more than one lock on it ('feck me - the blokes got a chain, a disc lock
and he's parked his car in front of the garage ... lets find another bike to nick .. '), think about security lighting, and maybe a cctv camera (even if it's a dummy).
Make it too much work. Make mateyboys bike in the next street an easier option ...