We have pretty good luck with a vinegar wash on the counter when we see ants. I don't know if the vinegar is actually repelling them or if just keeping the counters cleaner is the trick, but
something works. (I'm such a rotten housekeeper that I usually don't notice there are crumbs on the counter until I notice the ants.
)
I know of people who use diatomaceous earth to deter ants--sprinkle it near where they're coming in, or around the areas they seem to frequent. I've never tried it, but others seem to have luck with it.