1. Understand who your customers are:
Some of the biggest mistakes I've seen on sites are when they assume who the customer is... without actually find out the trust and validating that assumption. I'm sure we've all seen or heard of the exec who comes along at the beginning of a new digital project and says "I know who our customer is, I don't need to research them" or something similar.
Dip into your site's analytic package and see if this provides any insight into who your current visitors are (just don't assume this will be the same going forwards or that the same type of visitors all convert in the same way).
Note: If this doesn't tell you much, then carry out whatever research you can (Alexa demographic figures, online survey, site registration details, etc.)
2. Create customer personas
Personas are simply a way of describing the attributes, qualities and required outcomes of your key customer types. I believe that the aim here is not to be too descriptive about who they are (e.g. what income they have or what car they drive) but to describe their online needs (e.g. what they specifically want from your website).
Jakob Neilson says that you only need to test with 5 users to get the best from your usability tests [link] and I also think the same is true of personas. In other words, I recommend creating more than a couple but don't create too many.
3. Review your current site(s)
"Our site is rubbish, I want something entirely new" said the business stakeholder to me on a recent website redesign project. To which I replied... "that rubbish site is currently making millions of £'s worth of revenue for you each year, it must be doing something right!"
Yes, sure sites can always be improved... but just to slate everything about the current one without actually know what is working well (and conversely what is not) is of vital importance to ensure that you do not throw the baby out with the bathwater.
Carry out a set of user test on your website to see what users like & don't, plus where they incur obvious struggle in completing their goals.
Note: A few tests using a tool such as whatusersdo.com will highlight key user issues and also provide a permanent record of just how 'rubbish' your site is or isn't.