Yesterday I talked about adding tags to the web page addresses on your site from any adverts you run, to give you a proper look at the results from those adverts. But you don't have to limit this to the links from other web pages. Tagged links are also essential if you're running any sort of email campaign too: in fact, they're even more important.
Why is this? Well, if someone clicks through to your website from an email, what do you see in your Google Analytics report? An unidentified visitor. It could be anyone, from anywhere. What I normally hear from people is that "it looks like our email newsletter got read, because there's a spike in our site traffic on the day it went out". Seriously, are you really happy with that information? Just having (probable) confirmation that the email actually went out?
Which links in the email did people click on? Was the quality of the traffic any good? How much of the day's "unidentified" traffic really came from the email? What did those visitors do? Just make sure every link in your email has a set of tags, and you can measure everything about it. And yes, if you're buying a spot in someone else's emails, the same opportunity is there.