Just say it!
Say it loud and clear, with cheer, and for all to hear!
Don't be defensive. Take the initiative! Beat them to the punch line! Greet them before they greet you. See how many times you can say it in a day! Say it especially after the 25th of December (at least until the day of the Circumcision [octave day of Christmas: 1 January; and after until the feast of the Three Kings--6 January)! And, actually, 2 February is the final feast of Christmas (the Presentation of the first born in the temple on the 4oth day!). Christmas is 40 days!
Say it even after people think it's gone in the hopes to perplex and elicit questions to instruct about the octave day (his naming and Circumcision according to the Jewish law--New Year's is Christmas!) or about the Three Kings' feast (which is the day for presents in many national traditions.
Merry Christmas!
If someone says "Happy Holidays" look him intently and lovingly in the eye and serenely respond with a less cliche Christmas greeting. Say, for example:
"Jesus Christ is born."
"Emmanuel: God is with us."
"The Virgin conceived and bore the Son of God."
"Glory to God in the highest and peace to his people on earth."
"Praised be Jesus Christ."
"Long live Christmas and the Christ child!" (My personal favorite.)
