I have a huge colony of swallows in my barn every year. I don't love having them there, as per the poo factor, and I knock down any nests near my hay, but frankly I love what they do to the bug population, so I have ultimately (if somewhat reluctantly) decided while the pigeons get chased away, the bats and the swallows can stay. It is great fun to be followed by a flock of swooping swallows when I mow the pasture. Makes me feel a little Dr Doolittle or something. Lol
But as far as keeping them away, the only real tips I have ever gotten are as follows:
One way is to just keep knocking down the mud nests they build, and they will eventually get frustrated and move on. Note this has to be done before eggs are laid - while I doubt many people follow it, once eggs are laid, you can not harm the birds or disturb the nest, as swallows are protected by federal law.
I've also been told that placing fake rubber snakes in your barn rafters keeps them away.
A friend of mine had the best luck with painting her barn rafters with a garage-floor epoxy type paint. This might be a reasonable solution for you, since you are building a new barn from the ground up and can include this in the building plans - swallows need a rough surface for their nests to stick to. If all the rafters and such are painted with a slick coating, there is no way for the swallows to attach their nests.