I get this question all the time.
Most often the business owner or professional asking the question is looking for help with headlines or wording.
More often than not the writing is fine; its the approach that is all wrong.
If I give you the assignment to find and evaluate the websites of ten lawyers or ten doctors or ten accountants in your area you will likely find websites that look pretty much the same and that also say pretty much the same thing. These websites will usually talk in glowing terms about the accomplishments and experience of the site owner.
Experience and competence are important but from a marketing point of view they are not all important. Do you know where you accountant went to school or how he learned his business? Do you care?
What you do care about is your own needs. If you have a leaky pipe you want to find a reliable plumber who will come when you call and charge a fair price. If you are looking a computer consultant you want one who understands your business and knows how to help you be more efficient.
This is the key to good writing for your website: it needs to about your customer. You need to think about the problems of your future customers and tell them how you can help.
Once you figure that out the writing will take care of itself.