This is probably the single most important thing you should do, and almost no businesses do it properly.
It goes like this.
You have three ads going at once, one in the local paper, one on Facebook and one in the local doctors on a screen, all of which cost $2,000 each.
Bam, you have a great month on sales, but sadly you didn't test and measure, so you are faced with two options, option one, pay the $6,000 again and get all three running, or option two, have a guess which one worked and skip the other two.
As you can see, neither option one, nor option two are great options.
What if the scenario was more like this...
You did the same advertising, except you offered a different free gift for each one, or PROMO code, or even asked every single caller where they heard about you?
Now you have a list of where every sale came from, and if you made a note of the value of each sale, you can accurately work our your ROI (return on investment) for your marketing dollar. Most good marketing companies will try and deliver about $4 to each dollar spent, but I do see many now just offering $2 return for each $1 spent, which is quite low.
The point is though, how can you know what works if you don't measure.
The beauty of this is not just to see what works, but over time you can build up a list of marketing activities and be sure that each one will deliver results, whilst avoiding the ones that don't.
Many years ago I was handling the marketing for a company and we trialed cinema advertising, and as experienced marketers we tested and measured and found that there was almost zero ROI and made a note of it so we would never do it again. Then another client asked my advice on cinema adverting and I was able to show him how bad it was for his industry which was the same industry as the first.
Unfortunately he was adamant, and spent $11,000 on a three month cinema campaign, which he took my advice and measured properly, only to realise that not a single sale could be traced back to the cinema advertising.
My message is not that cinema advertising is bad, the message is that if you don't measure you will not know what worked and you will waste money. Imagine if he did a PPC campaign at the same time and it rocked and he didn't measure either, he would have signed back up to the cinema advertising and wasted another $11,000.