I am a bit OTT about security, but it is discrete.
Garage has an up and over door, a wooden side door and a single glazed window. Both the side door and window were beefed up internally with bars secured to the frame and surrounding brick - only visible from the garden. The up and over has a heavy duty hasp lock as well as a shoot bolt in addition to the pathetic lock they come with.
Inside the bikes are mostly d-locked to the floor - or cable locked to wall mounts.
That said, someone did try to get into my garage last year, removed the glass from the window, but was met by the window bars. Didn't get any further. I did, however add additional bars and a shed alarm.
I can't get my best bike replaced - it's custom built and the chap that built it for me is no longer with us.