In a previous post, Kurt was talking about how ads could be put in his invoices. He has space on the invoice for them. You should constantly look at these things and continue being creative.
Take for instance Apple. They are now going to include ads in their iTunes program!
AdAge.com ** Coming soon to iTunes: ads. Apple ** a brand that prides itself on the purity of the user experience ** will soon put up billboards on its popular iTunes service, according to content partners who have been briefed on the plan. The introduction of visual ads could be the first step to allowing ads in other content areas or on iPods.
And why not! What other ways could you gain more income by using ad space?