We all have so many ads pushed at us daily, that we've become somewhat used to them. They're everywhere, so it's no big deal to include them in your site, especially if it brings in some extra money, right? Think long and hard about including ads on your site. Ads can not only cheapen your site, but they can detract from your own business's products and services - which is what you're trying to promote. Often ads can do more harm then good if they don't bring in enough revenue to justify turning away potential customers.
