I like to keep mine in a decent sized Moleskine notebook because I like to write things by hand (and it makes it easier to use self-created symbols to mark trends). However, you could also keep a binder and print out your pages or you could probably make an Excel spreadsheet, if you're good at that kind of thing, but using Excel is going to make it harder to keep notes in the quantity you'll need.
First and foremost, I mark the first several pages for Notes only, so I can write down things like symbols I use, marketing websites, popular items in my shop, or whatever else you might want to note separately.
Second, I began to date pages, keep the top half blank and marking a section of the bottom half as "Results." Under results, I then marked three more sections: Google Analytics, Etsy and Other. In Google Analytics, I have six selections: Visits, Pageviews, Pages per Visit, Bounce Rate, Average Time on Site and New Visits. Under Etsy I marked: Shop Hearts, Item Hearts, Number of Sales (# Sales), Dollar Amount of Sales ($ Sales), and Circles. Under the Other Heading I put: FB Followers, Twitter Followers, Blog Views and Blog Followers. Of couse, you can edit this to fit what marketing outlets you use.
Now, I keep this journal with my computer and whenever I do anything that might have an effect on my shop (relisting items, spending mass quantities of time in the forums commenting, linking to my shop on Twitter, making a blog post, building a Treasury, to name a few) I make a note of it in my journal. I also have shorthand abbreviation (FB for Facebook, TW for Twitter, K for Kaboodle, etc), that I use to save space (all of which are marked in the Notes section at the beginning).
At the end of the day (I stay up late) or the next morning, I then gather all the data, using color-coded pens. Green symbolizes an increase, red a decrease, and gray means no change. If a number has risen for two days, it gets an arrow beside it pointing up. If it's risen for three or more, it gets two. The same goes for falling numbers, but with the arrow pointing down.
The whole process doesn't take more than five minutes each day from start to finish and now I can keep track of what is making a difference in my shop traffic! Just keep in mind that it's important to keep up with everything. I will often make notes beyond my numbers if I see in Google Analytics that I got traffic from a specific source, or if I was featured in an unusual amount of treasuries that day.
Hope you find this useful in bringing success to your Etsy shop!