For marketing methods such as television or print advertising there exist well established measurements. You know that spending $X advertising on certain television shows, with particular networks, during specific hours of the day or times of year, in magazines based on their readership that Y number of people will see your advertisement. By tracking how much you sell before, during and after the advertising runs you can develop a recipe for hitting your targets.
For instance if you know that spending $1 million on television advertising during the two weeks prior to Easter leads to sales of 1,000 of your custom egg baskets you can include that in your plan. Perhaps you would choose to do that every year. Possibly you might find less expensive ways to drive those 1,000 sales. Or maybe after those 1,000 customers find you they are loyal year after year and you can re-deploy your $1 million to other areas of your marketing or business plan.
Another way to think about measurement is to look at how much extra you get with your marketing efforts. For example, if you sell aprons on Etsy and without doing anything beyond listing the aprons for sale you sell 20 aprons a month. Those 20 sales would be your baseline. Let's say for marketing purposes you offered a discount code on twitter and that brought in 5 additional sales. Those 5 sales would be incremental sales. Comparing differences in base versus incremental sales volume can give you rich insights into how to adjust your marketing and branding efforts.
Even if you are not marketing a product or service, tracking key metrics can still be a useful exercise. Perhaps you want to increase your blog readership. You might want to track page views by subject, which topics people add to social bookmark sites or on their Facebook page or posts that readers are moved to Stumble or Kirtsy. Looking at those numbers can lead you to fine-tune your writing and topic selection in ways that increase readership and engagement.
Regardless of your goals, establishing key metrics related to your goals and then tracking and measuring them will help you both better understand the effectiveness of your efforts and reach those goals.
Pick a few (1-3) metrics and then track and start to measure. The information you get will be very helpful and give you a much fuller picture of how effective your efforts are.