Encouraging them to nest
House martins need three things to help them breed successfully a suitable nesting site, the correct building materials and plenty of food.
What you can do
As house martins feed entirely on insects caught in flight, they cannot be attracted by providing food. A muddy pool or puddle where they can collect nesting material is helpful, especially during a dry spring. They appear to choose sites at random, but painting the top half of a building white or cream may attract them.
Various artificial house martin nests are available. These are usually mounted on a board and can be fixed easily under the eaves. They do not guarantee that martins will nest, but often encourage them to build their own. The artificial nests are best placed in groups, especially near existing nests. There is some evidence that martins prefer to nest on north and east facing walls.
Artificial nests are commercially available. Alternatively, you can make an artificial nest from exterior fillers such as Polyfilla or a mixture of cement and sawdust, using a quarter segment of a plastic ball about 180 mm in diameter as a model. Cover the model to a thickness of about 8 mm, leaving a flange around the edge to aid fixing. The entrance hole should be a semi-circle 25 mm deep and 60-65 mm wide.
The nest can be mounted on a board for fixing, or fixed directly to the house with exterior filler. It helps to smear the outside of the nest with mud, especially around the hole.