Each matchmaking system has different ways of searching for compatible profiles. Some have several ways. Typically these are MATCH, SEARCH, FIND, and some allow you to make a Search Profile that you can reuse.
You can search by location, ideal height and weight, ethnic background -- even by interest in having kids in the future. A high percentage on a multiple choice match is a good start but read the answers and look at the essay.
The percentages may be great but then the profile might be someone who sounds desperate and lonely, is someone just looking for sex or has totally different interests and values.
If there is a photo, look at the photo and remember that the photo can be one taken yesterday or 5 years ago. Unless the person is down right unappealing, remember that amateur/family photos don't always make a person look their best.
Look for profiles that match your ideal characteristics but also hit you the right way -- do the writers sound funny? Intellectual? Love animals as much as you do? Like to travel?
It is up to you to post a photo. You will increase your response rate TEN TIMES by posting a photo.
People want to see who they are writing to, and many don't want to start a correspondence and waste time with someone that they don't know if there is even a initial attraction from a photo.
If you want a lot of responses, you'd better have a picture. From my experience, and from what I've heard from others, it seems that people who don't have pictures of themselves are usually hiding something. So, if you don't have a picture, people are going to assume you look like a dog.
If you are a high profile person in the city you live, offer in your profile to exchange photos from your personal (yahoo or hotmail, not your real email address). Make sure you put an accurate description of what you look like in your profile. You may want to say what celebrity you closely resemble.
People who say they don't have a photo or don't have a way to get one on line are either lazy or playing games. If you don't have a scanner at home or work, take a photo to KINKO's (they are everywhere).
Have your photo or photos scanned in a .jpg format. Most matchmaking systems do not allow you to send the zip files or unusable formats. Typically .jpg, .gif, and .bmp is the limit and they must be sized down. Photos should have a shirt on, clearly show your face (no sunglasses), well lit, no swimwear (except for secondary shots) and no family in your primary photo.
You can also read on Choosing an Online Dating Service