Quantifying your marketing effort

It's true we spend marketing dollars to display at trade shows, to attend events, to hold conferences, and to produce marketing material for campaigns. How do we know what we are getting in return? How can we quantify the results to make sure they are worth the revenue spent?
This may seem like an easy question, however it's one that I am asked often. I have seen companies that don't even quantify their marketing efforts. Let me just say that's a big mistake. While marketing can be in the most part trial and errors you can diminish errors by actually using calucations to see which campaigns are bringing in the most results for the money.