If someone wants your bike, they're (probably) going to get it. Other than that, as vickster says, get a really solid lock, lock your bike up properly to something immovable and make your bike as inconvenient to steal as possible, so hopefully they'd move onto an easier target. Also if it's "secured" in a place where a thief isn't going to be seen or disturbed, then it may as well not be locked up at all.
That said - contents insurance can be great - I think mine covers me for theft and accidental damage both in and away from the home, and from within a vehicle - provided it's not just left around unattended and unlocked. I believe that added about £25 to my annual policy costs, up to £3500 worth of a single bike. None of mine are worth anywhere near that. But fortunately I haven't had to test whether or not they'll pay out for anything. And I never leave it unattended anyway, locked or otherwise outside of a building, but that may change if I have to leave it in the sheds outside work.