Decide who your target audience is going to be. If you are only looking for casual encounters, then do not mention a long-term relationship in your tagline. The tagline of your dating profile is the first impression you are going to make, so be sure to know who you wish to target. This promotes a sense of trust with potential matches if you are honest from the beginning.
Determine what you want to say about yourself in one phrase. What is the one thing you want potential mates to know up front -- your best quality and how you want to attract a woman are all important points to consider in this step. This, plus knowing who your target audience is, will help you determine how successful your tagline will be.
Practice writing your tagline before typing it into your online dating profile. Try several different versions to see which one fits your needs the best. Also try to run several taglines by people you trust to see which one they think would be the best fit for you. This is important to do because it always helps to have different people's perspectives.
Write your tagline. Once you have done this, read over your entire profile before saving the tagline to double-check that the tagline is accurate and fits the actual body of your dating profile. Again, it boils down to being honest from the beginning.
Enjoy the fruits of your labor. Correspond with people you feel are a good match and were attracted by your tagline. Perhaps your tagline will help you meet Mr. or Ms. Right; no one can say for sure. Have fun and remember to be safe.