If you're going to improve your website, start by deciding exactly what you want to improve, so you'll know when things are getting better. Be specific about what you want visitors to do on your website. For example, do you want them to spend longer on the whole website, or just on certain pages? Do you want more contact form submissions, or only those from certain prospects?
Whatever you decide, track it for a few months before you make any changes so you've got something to compare.
Get the basics right
You may get the greatest change by looking at the basics of your website. Before you worry about USPs and calls to action, try a user video service like www.whatusersdo.com to see how people encounter your website for the first time. You can set a simple task and watch a video of a user's screen with their commentary as they try to complete it. If you know your website well, you may be surprised by what confuses an average user.
Catch them at the door
If you had a shop, you could stop your customers and ask them what they want (or can't find). The closest equivalent on the web are pop-up surveys like those provided by www.qualaroo.com. Add a simple snippet of code and you can start asking people why they're leaving. Choose whether to ask the question immediately, after a set amount of time, or when a key action is taken.
By asking a simple question like 'why are you leaving?', you just might get a few ideas of what you can improve. Another option is to add live chat to the website, so you can answer those questions directly.
Explain the benefits
As with any marketing, a website sales process depends on clearly stating the benefits of choosing you over the alternative. Just the act of clearly defining two or three benefits and repeating this on your website could have a significant effect on whether people stay long enough to get in touch.
The important part is finding what benefits matter to your users, as they may not be the ones you think. Try an email survey tool like www.surveymonkey.com to ask your existing clients why they chose you, or talk to your sales and customer service staff.
Once you've planned and applied your changes, make sure it's working by reviewing the indicators you chose to track at the beginning. Real improvement may take time, so keep trying new things and testing against your indicators, and treat your website as a constantly evolving tool.