DEAR ABBY: My father taught my brother at a very early age that on being introduced to someone, he should always look that person straight in the eye and deliver a firm handshake. "It's a sign of good character and sincerity," Dad used to say.

Is that right, Abby? I hope not, because our son-in-law has one of those "dead fish" handshakes, and he rarely looks a person straight in the eye. - RICHMOND, VA.DEAR RICHMOND: A firm handshake and the practice of looking a person straight in the eye make a good first impression, but it's not a reliable index of one's character or sincerity. There are people who can deliver a firm handshake and look you straight in the eye while they lie through their teeth.

Character and sincerity are revealed only by PERFORMANCE.

DEAR ABBY: I have never written to you before, but I really need your advice now. I can't give you my address because I feel like a fugitive. Here goes:

My husband and I have never paid taxes. We are in our early 30s and have been working since we were 20. When we were very young, we were ignorant about how to pay taxes and file and such. Then we just got careless and never filed. Now we are afraid to admit that we never filed because we fear the consequences. We are born-again Christians and don't feel right hiding from the law. We want to make things right, Abby. Please help us. Where do we begin? - ANONYMOUS IN THE USA

DEAR ANONYMOUS: Get in touch with the Internal Revenue Service. It is listed in your telephone directory. You can expect to be penalized for your failure to file, but if you were to come forward voluntarily, you would receive more generous treatment than if the IRS "caught" you. The longer you wait, the more severe the penalty, so call the IRS and PDQ.

DEAR READERS: A timely reminder: Next Sunday is Mother's Day, and here I am again with a suggestion that could be the most appreciated Mother's Day gift your mother has ever received. And the price is right.

It makes no difference if you are 8 years old or 65; if you are lucky enough to have a mother, sit down and write her a letter. It doesn't have to be a literary masterpiece - just a few sentences telling her how much she means to you, and why. If you are in a sentimental mood, go ahead and verbalize some thoughts you've felt, but never expressed before.

And be sure to date it, because long after the nightgowns, purses and hankies are worn and discarded, I'll bet your letter will be tucked away for safekeeping with other cards and letters your mother just couldn't bring herself to throw away.

How do I know? My letters were. - ABBY

Everything you'll need to know about planning a wedding can be found in Abby's booklet, "How to Have a Lovely Wedding." To order, send a long, business-size, self-addressed envelope, plus check or money order for $3.95 ($4.50 in Canada), to: Dear Abby, Wedding Booklet, P.O. Box 447, Mount Morris, IL 61054. (Postage is included.)