Place a sign next to the item you want to protect and type in the following

Line 1: [Private] Line 2: Your name will be automatically filled, users with "deadbolt.admin.create" can specify someone else.
Lines 3 and 4: You have a couple options for these lines
Another player's name
Unrestricted access via [Everyone] while preventing breaking
Create an automatic timed door using [Timer:1] through [Timer:9]

Not enough room for all the names you need? Make another sign with the [More Users] on line 1
For ease of use in maintaining your signs without having to break them:

Right click the sign to select it.
Use "/deadbolt <line number> <text>" to directly modify that line. When placing signs, valid locations are to the NORTH, SOUTH, EAST, and WEST of the target block.

Other valid blocks include:

Doors: The blocks above and below.
Trapdoors: The block that it is attached to (hinge-block) and directly above/below the trapdoor itself.
Fence gates: Any block horizontally adjacent to the gate itself.
Use your imagination and hide those unsightly signs under walls.