There are three core stats you should be tracking on your sign-up pages. These three stats build on one another, are easy to understand, and give you some goals implied goals.
1. # of referring sites: This tells you how well you are doing at placing references to your business around the internet. You want to work on driving this number up with word of mouth, SEO, and targeting online customer communities/blogs.
2. Views: This tells you if people are getting a chance to see your message. Is this going up or down over time? What have you done lately to market your site? The first goal (referring sites) plays directly into this.
3. Signups: This is how many people have signed up for your service, for more information, or your mailing list. You then want to measure this against your view counts to understand your conversion rate.
There are a lot of other metrics you could get into, but these are the most basic and give you the most accurate picture of your success.
From these numbers you can tell if you need to tweak your message because you have low conversion rates or if you need to work on getting the word out online because you have good conversion rates, but low view and referral counts.