This will sound like I am taking the piss - I'm not though!
Stand the bike on it's back and wheel and get a friend to take photo of your face next to the frame number. Keep this photo in your mobile phone.
Sadly if you're bike does get nicked, there's no way you'll get it back really. Prevention is better than a cure.
But the idea posted above is a good one but a better one is to take a picture of you with it as mentioned, photocopy that picture, write a parts list on it, roll it up and stick it inside your seat post. So if you do catch them at it and there's the po po around to intervene you can prove it quick and easy with an allen key. But you'd be lucky to be in that situation...
Sadly if you're bike does get nicked, there's no way you'll get it back really. Prevention is better than a cure.
But the idea posted above is a good one but a better one is to take a picture of you with it as mentioned, photocopy that picture, write a parts list on it, roll it up and stick it inside your seat post. So if you do catch them at it and there's the po po around to intervene you can prove it quick and easy with an allen key. But you'd be lucky to be in that situation...