Not a question with a simple answer so first I'll point you to a couple of sites that talk about conversions a lot
grokdotcom - If you see anyone writing about conversions the thought probably originated here. Read the blog, look though the site, buy their books. They talk about what they call persuasion architecture. All good stuff.
Invesp - Khalid, the person behind this site is a friend I met through the old forum. Their whole site deals with conversions and is also all good stuff.
The first thing I would ask is what do you consider a conversion? Is it adding something to a shopping cart of getting someone to email you to contact you about your services? Is it someone subscribing to your blog or clicking to view a specific page? There are many things that can be a conversion so the first step is defining what that conversion is.
For some things it might be as simple as trying a different color on a button or making a button more visible. Maybe changing the text on the button. Usually if you want someone to take an action you need to tell them what that action is. It's why you see 'click here' everywhere. By giving the command to click you get more clicks.
Some of it is understanding that different people respond to different things. Some people need to know others have used your services so for them it's important to have testimonials on the site. Other people could care less what others have done and just want to know the details of what you have.
Some is understanding people and what makes them take action. People generally don't care why you want them to do something. They want to know what's in it for them. It's where the talk of benefits over features comes from. Your visitor doesn't really care that your websites are coded well. They want to know that your code makes the page load quicker and keeps people on their site. Give people the reasons why your services/products helps them in a way they want to be helped.
People act more on emotion than logic. One powerful emotion is fear of loss. That's why you'll see offers with deadlines or limited offers. People don't want to miss out and the nature of the offer says if they don't act now or soon they will miss out.
Greed is a powerful emotion. If you can convince someone that hiring you will make them more money it can convince them to buy from you.
I've certainly only scratched the surface. Do check the blogs I linked to above and let them lead you to more info. I definitely recommend the books written by the Eiesnberg brothers at grokdotcom. Click on the Resources link in the menu. All their books are available at any bookstore and Amazon.