Check Your Spelling
Write your personal ad with word-processing software such as Microsoft Word. If you do not have Word, you can use Google's Docs feature free of charge. Use the software to check your ad for spelling and grammar mistakes. An ad should be a representation of you at your best and is similar to a resume in that it makes a first impression. Do not make the wrong impression by making simple mistakes.
Show, Don't Tell
Many people write the same exact things in their personal ads. They describe themselves as "funny" or "fun." While you may be funny or fun, try showing your readers this instead of telling them. For instance, if you perform in a sketch comedy troupe, that shows readers that you are probably funny and serves as an example of one of your interests. Show how your personality and outlook influence your life, and your ad will get better.
It can be tempting to fluff up some of your accomplishments or cover some of your more disagreeable opinions in the spirit of getting more responses. However, this will almost always come back to bite you. Being honest up front about who you are may bring you fewer responses, but those responses will be better since they will be from people interested in the real you. Be honest about your appearance as well -- fibbing here will be quickly spotted on the first meeting.
Be Fun and Interesting
People reading the personals section of a newspaper are usually looking for interesting people that can show them a good time. A bland ad shows that you are likely a bland person and does not stand out from the crowd. Use your sense of humor to create a funny ad that expresses how much fun you are in person, and the reader will be able to better imagine spending an evening or two with you. Just be sure to keep it tasteful.