How do I prevent spiders from accumulating on my windows?

The best answer I've found is to take a broom to their webs on a regular basis, say every couple of weeks in the summer and monthly the rest of the year.

The rationale behind this is fairly simple. The spider spins a web in an area where some air currents run in order to catch flying insects to eat. If the web is broken, no dinner.

The spider will try to catch dinner someplace else, like the neighbors' or the woods.

Another reason is that it makes the house look better with no cobwebs or other Halloween decorations.