How To Keep Spiders, Ants, Wasps from Bird Houses?

Please share your tips on how to keep the critters from invading our feathered friends' houses in the garden.

I've not had ants or spiders in my birdhouses, but a couple of wasps early in the season. I rubbed bar soap (Ivory, if I remember) up on the roof of the box.

Would spiders be a problem? I think I remember reading that House Wrens put spider egg sacs in their nests. I believe the theory is the baby spiders eat the mites. I just read on another forum, some think the birds would eat the spiders.

Thanks CB.