If you like the idea of bird boxes but not the prices at your local outlet this RSPB webpage has a DIY design (from one of their leaflets):
http://www.rspb.org.uk/advice/helpingbirds/nestboxes/smallbirds/making.asp
Apart from the "where-to-cut-the-plank" details the other instructions are worth knowing. The bird box in our garden has no drainage holes and cannot be opened for an annual clean. No wonder the birds ignored it this year and let the wild bees move in.