I selected a"Sunny" white balance to get the crisp clear blue. Also remember to set the focus point on the silhouettes and underexpose by 1-2 stops, to ensure that you get sharp silhouettes and avoid a burnt-out sky.
For the second-to-last shot I have converted to b/wh and then tinted the image.
The last one is only converted to s/wh. Simon was my model here and since the picture is taken from a lower level I asked him to come a bit closer so that I actually could see his legs.