What marketing investment offers the highest ROI for small businesses?

Get the budget and money approved for some small swag, get into a cab or fly out if you have to and visit your existing customers. There is no greater reward in client loyalty and retention. See if a possible up-sell opportunity arises, or if they can provider a referral.

So many a times we tend to ignore or become oblivious to the best avenue for revenue that we already have, our customers. Don't ignore them. Invest in them and you will be able to gauge the results very well.