A properly fitted battery cut off switch makes a very good safety and security accessory. There are two ways in which it can be done, the first being the most simple and the second slightly more complicated and by far the better of the two.
The first is just to fit a basic isolator switch in the main +VE cable between the battery and the starter-motor common terminal (one of the leads will probably need extending so make sure you use the correctly rated cable, especially if you are extending the run). Mount the switch so it can be reached by the driver and remove the key whenever you park-up but DO NOT switch off the switch when the engine is running or you will damage the alternator.
The second way is to use a competition switch (pictured above) which incorporates two pairs of terminals as well as the main battery lead terminals. One of these pairs of contacts opens when you turn off the switch and kills the engine by either interrupting the ignition supply (petrol) or closing the fuel solenoid (diesel). The second pair of contacts closes when the switch is turned off and this diverts the power from the alternator (which is still producing power until it stops turning) and diverts this power through a ballast resistor to earth thereby protecting the alternator.
If you have accessories which require a permanent feed (radio, alarm etc) you can run an aux fuse box direct from the battery to power these items.
Both these switches are available from car accessory shops, motor-factors, race/rally specialists etc, the basic one is about £10 and the competition one is about £22 (both include 1 key but spares are only about £1 to buy). Some online suppliers include the following