Chose a tone. You obviously wouldn't write the same bio for your comedy blog as you would for your business website. Choosing a tone allows you to focus on what information is appropriate to include, and what isn't. It also creates a natural starting point, eliminating the "how do I want to do this?" mentality.
Decide how much personal information you are comfortable giving. Some people are open books, while others don't even want the public to know what city they live in. Determine the possible impact that any personal information you give will have on you, your family or your business.
Make a list of key points you want to include. Do you want to include as much information about your business as possible, or do you want to talk about your family and hobbies? Is your bio going to be a marketing tool or simply an introduction? A list of things you would like to include is the rough outline to your biography.
Write a draft, paying no attention to word count. This is just the "getting everything out" stage. You can go back and edit once you have your bio down on paper. Start with the list you made and write a sentence or two about each item.
Edit your draft. Remove any items you think are less important than others and resequence your sentences into a more logical order. Next, add transitional words or phrases to make your biography sound less like a list and more like a flowing piece of writing. Finish by editing for spelling and grammar.