Flattery will get you everywhere, even with people who wouldn't admit it. "You're pretty" is a compliment, sure, but it's not a very memorable one. "Your lips are better than Angelina Jolie's," while not very likely to be true, is a much better and more specific compliment.
And remember that compliments don't have to be based on appearance! Showing appreciation for a profile that's free of misspellings, grammatical mistakes, or punctuation errors is likely to get you points with the person who went to the trouble of creating a perfect self-description. Or, compliment the person on an unusual combination of characteristics: "I've never known any football players who also read Gore Vidal—how'd you get to be so unique?" Specificity is key, as is actually complimenting the person. (Hint: "You're better-looking than my ex" is not the right phrase for kicking off a new relationship.)