A few sites that helped me when I was starting to draft my own SOP were Katherine Sledge Moore's Statement of Purpose guide, and a humorous article on Kisses of Death in the Graduate School Application Process. Both of these refer to psychology essays, but the key lessons remain true for the life sciences as well (especially neuroscience, which seems to attract students with psychological disorders just as well as psychology -- don't use your personal problems as your explanation for why you want to study the brain!). For another humorous take on the SOP, check out Female Science Professor's purposefully terrible essay (prefaced with real useful advice).
When it's time to start working on your SOP, I suggest writing a long, rambly draft as a form of brainstorming. Tell the story of your academic career, describing the parts that are significant or unique. If you were inspired to become a scientist because of an amazing class you took on evolution that involved field work and led to a really cool final project, write about that. If you learned some lessons about the importance of trouble-shooting by spending months perfecting a PCR, write about that. Write about your undergraduate advisers and what you learned from them. Write about presenting a poster at a conference and how awesome that was.
Now, look back on your rambly draft and try to highlight the most important things in it. The most important part of your graduate school career will be spent in the lab, conducting independent research, so you want to show that you're prepared to do this. If you already have lab experience, you should definitely write about that! Talk about your research projects, and make sure you explain why you did them, in addition to summarizing the methods and results. It's great to talk about more than one project, if you've had the opportunity to study several different things. But, if you don't already have lab experience, you'll need to show that you're prepared to succeed in the lab as a grad student. Write about significant research papers or group projects that you completed as an undergrad, if they are relevant to your field. Share an anecdote about your problem-solving skills, or your ability to organize large amounts of data into a cohesive argument.
Some people also use the SOP as a space to explain any problems with their application, but I disagree with this approach. Don't waste space making excuses for your past mistakes; focus on the positive. Show that you're prepared to do a great job when you get to graduate school. If something really significant impacted your undergraduate performance, this is better addressed by one of your recommendation writers, who will be able to provide a more objective take on the situation and vouch for your ability to succeed after resolving those extenuating circumstances.
Once your rambly draft has been cut to a more specific essay focusing on your best scientific strengths, you'll need to make sure that you include your purpose for applying to graduate school in general, and to each school specifically. I think it's best to include the general statement of purpose at the very beginning of the essay, since the essay is called a statement of purpose, after all. Refer to your long-term goals, explain how you came to have them, and show that attending graduate school is part of your plan to achieve those goals. For many people, the main goal is to become a professor at a major research institution, but that doesn't have to be the case for you. You can be more vague, saying that you know you want a lifelong career in science and that a PhD opens the door to myriad options in your field. You can state a specific goal of working in industry, government research, public policy, or biotechnology patent law, if that's what you're into. Some may caution you against stating an "alternative career" goal, since the people reading your essay will be professors. Such people may think that being a professor is the only appropriate thing to do with a PhD. I think if you can explain your reasons for wanting a different career in an intelligent and passionate manner, you should go for it. It'll make you stand out! But if you're not sure what you want to do, it's okay to write about less specific goals. No one will hold you to what you said in your SOP when you're looking for jobs years down the road.
In addition to your general statement of purpose, you'll want to write a "fit paragraph" showing that you've done your homework on the schools to which you're applying. This usually comes at the end of your SOP. After describing yourself and how great you'll be as a graduate student, you describe why that particular institution is the place for you. Some things to mention might include the school's high standards for student research, commitment to excellence in a specific area of science where you want to work, and supportive environment for training new scientists. Referring to aspects of the curriculum that specifically address your goals is a good way to go, as is mentioning any research facility dedicated to your area of interest. You can also mention specific professors at the university. If you do talk about individual professors, I suggest dropping several names. If you mention only one, and that person isn't accepting new graduate students for whatever reason, then your application may be discarded as a bad fit. If you mention several professors, you give yourself multiple options. Overall, the point of this paragraph is to show the university how they can provide what you need to achieve your goals, and how you fit nicely into the program given its academic and scientific priorities.
After you've transformed your SOP into something resembling the real thing, have someone else read it for you. This person doesn't have to be a scientist, just someone with good writing skills. I happen to live with a talented English major, so he read my SOP and gave me helpful feedback on how to make my writing clearer and more concise. A friend, a mentor, or a volunteer at your university's writing center can help you with this. Take their advice to heart and produce an edited draft. Repeat as needed until you get confirmation of a job well done.
I've uploaded my own SOP for you to look at, to give a concrete example of many of these points. This SOP was written for my application to the Neuroscience Graduate Program at Emory University. Obviously, this essay is presented for educational purposes only, and it would be very foolish of you to copy any part of it for your own applications. That sort of academic dishonesty can get you kicked out of school. Refer to the essay and my added comments as a guide, but showcase your own achievements using your own writing talents to get the point across. (Don't mind the comments attributed to Jamie Lay. Jamie is my uncle; he gave me a used computer with the accompanying Microsoft Office suite still registered in his name.)