The best height for your nest box is widely accepted as being between 1.5m and 5.5m high (5ft - 18ft respectively). However, if your area has a particularly high cat population it is best to site your box even higher.
If you only have an exposed site to offer, face the box somewhere between north through east to southeast, avoiding prevailing winds and strong sunlight. If siting in woodland, the dry side of the tree trunk offers the most protection. By their nature, open nest boxes require more cover; siting them near to climbing plants where they can be partially obscured is ideal. Siting your nest box near vegetation also aids young birds taking their first flights as it gives them both physical support and good cover.
A clear flight path into the box works best and avoid sites such as the top of fences that make it easier for predators to get at the box.