They'll get it if they want it.
If it's somewhere public, make it very public. Once I parked my bike at Lakeside in a multi-story carpark... while I was shopping (about an hour, at about 8pm-9pm), someone unbolted my fuel tank to get at my electrics and only left it when they found a Meta alarm where they thought the normal loom would be... Then I took a job at Lakeside, and in the five months I've been leaving it in plain sight in a main thoroughfare that's always got people around, it's not been touched.
If it's at your house, just try and make sure no one knows it's there. i.e don't leave it out - when you get in, put it away. If it's out of sight, they won't know it's there, and if they don't know it's there, they can't steal it.
The stories you hear about bike thefts are absolutely shocking, if they want it badly enough there's literally no stopping the scumbags.
Besides that, an Almax lock attached to a ground anchor that's attached to something pretty ****ing solid.. you know, like.. the planet. As above, try and keep the chain off the ground, to help fend off brute force attacks. Zanx's videos of him chopping through chains that cost £££ in under a minute is pretty scary... makes you realise that if they want it they'll have it, so when it comes down to it, you best security at home is obscurity.