It depends on the age and whether the party is for an adult or child. Here are some ideas. "Sam is turning 1. Help us celebrate!", "Sam is turning one, join us for some fun in the sun, let's celebrate!"
Ideas for birthday invitations wording should definately catch the eye of the person reading them. A banner style announcement is usually effective in captivating the attention of the reader. A headline such as "BIRTHDAY BASH" should be adequate.
There are lots of Web sites that offer specific wording depending on the age of the celebrant and kind of birthday party. It's always fun to play on popular advertising thems. Consider "A birthday is a terrible thing to waste..." or "Candles--$1.00, cake--$13.00, Bill--priceless."