The dating services reviewed on this site all allow the ability to place a personal ad for free.
When filling out a personal profile, you are usually presented with an introduction line (a personal quote) that sums you up. For instance, 'Irrestible and attractive Graduate, looking for Love in San Fransisco' might be ok, but 'no wierdos or jerks, okay!' isn't going to have singles furiously clicking to read your profile...
Profiles contain a section that allows you to describe yourself and also what you are looking for in other singles, but don't treat it as an essay. Anywhere from 50-300 words is advisable.
The key to personal profiles is to be totally honest in what you write. Know yourself first, write down a list of things you like and are important to you. It may take a while but is well worth it. Use Eharmony to help you here.
Try to avoid using the cliche words like trustworthy and honest in your actual description. Similarly, try to prevent the urge to use the newspaper style of personals ads - 'SWF GSH 35-yr old seeks partner...' - yeah, that describes you and a few million others. Make the most out of online dating by using it's time and space advantages when creating your ad. Construct it in a clear, concise manner with a touch of originality, directness and personality.
Once you've prepared it, you can then adapt it and use it on other sites you join. It also helps when describing your attributes to provide a short example, for instance if you are a dedicated individual, why so? Well, you might 'dedicate time to fostering injured animals' or 'are an active member of your local charity', etc.
Once you've entered your profile, or before you do so (if the site allows), browse the site and read several profiles, which will give you some ideas of different styles, and what works. But try to be creative and unique if you really want to stand above the rest. Write your profile in such a manner that it appeals to people - if you're passionate about something include that, but be specific. Include humor or wit, ask a couple of friends/family to read your profile. Double-check it for glaring spelling mistakes and write in a positive honest manner. Dishonesty catches up with you in the end.
Additional sections are sometimes available that allow you to specify a variety of characteristics: interests, hobbies, education, career area, religious beliefs and so on. Some questions are open-ended, but I advise keeping the answers short and to the point as this makes it simpler.
If you feel comfortable, and want a better chance of getting a response many sites also offer the free ability to post your picture with the profile, and some even allow a short voice and video greeting. Many people only browse those profiles with a picture, so it's well worth considering. And that includes a recent picture, not one of when you were an 18 year old...when you're now a 30-year old.
You shouldn't restrict your dating search to one site. Diversity, and try maybe 3 or 4 services, and communicate with several people to increase the chances of finding better potential matches. If things don't work out, persevere.
It's important to realise that many people will join services, and only check them occassionally, and many join and forget about them. Some sites give you an idea of how many active users there are which is helpful.
Your chances of finding someone special will increase the more you persevere - the more experience you gain in online dating services, the better your replies and personals will become. The more profiles you read, the more ideas you'll discover for ways of improving your own descriptive profile.