You need to determine the metrics of success for your business, then what drives them, then design your site to assure you can achieve them efficiently.
As an example, if you wish to have people register for a free service then later subscribe via PayPal to a monthly subscription to obtain additional features (a freemium model), you should plan out an optimized registration process and build it into your site as soon as possible.
Therefore, where do you promote the registration and how. What is the call to action, the registration path, and the path of least resistance in terms of collecting information to assure high conversion rates. Define that, build it in, and create your reporting.
Do not let reporting become an afterthought. Reports on your key metrics should be built in. How many registrations do you have per day, how many subscriptions, how many unique visitors, how many went to the registration page, how many converted, what percent. Then you can test once you have basic statistics. You may have to generate some information from your system and add information from a statistics package such as Google Analytics.
At the very least please determine what your metrics are, just take a look at your business plan. You can usually look at 1st year projections, or 2nd and 3rd and back into quarterly, monthly, daily goals. Then determine what metrics from your site/service need to be tracked to determine the progress in relation to the goals. Next create a simple reporting interface, then comes your dashboard and now you can run your business.
Start the above as early as you can and as early as makes sense, not 90 days after launch when the board questions your progress only to scramble and throw the office into chaos mode.
In my past 10 years of online start-up experience I have seen too many start-ups spend 90% of their time on development and features only to later realize that they need revenue and reporting on metrics to evaluate their business progress.
Take the time to evaluate your business metrics and design them into your site as well as create the necessary reporting to gauge progress.
Ooh yeah and don’t forget revenue 😉