In my experience (as the best marketing consultant on the forum

) I would say that the biggest mistake people make is not really thinking about what marketing is. Probably 75% of new clients think that marketing is all about advertising. A few understand the principles a bit more and include research as part of their marketing strategy.
My first job is always to convince them that marketing should be used from the ground up, in any business. Such things as product selection, product development, selection of staff, targeting, web design, and even such things as selection of equipment can all be most successfully thought of from a marketing perspective. Anything you do in your business, that effects your customers or potential customers, is worth considering from a marketing perspective.
If more people knew this, they wouldn't even need to think about the expensive nonsense often passed off as marketing.