• Announce whose birthday you are celebrating on the invitation
• Include a "Happy Birthday" banner or sign at your venue
  
• Add a few extra theme-related festive decorations, such as streamers or ceiling swirls

Add some special events during the party:

Cards & gifts:  You may want to take a few moments for your birthday boy or girl to open cards and gifts.  Most of your guests will want to see his/her reaction, however, it is not always necessary. 

You may choose not to interrupt the momentum of the party, especially if only 3 out of 30 people brought a card or gift, or you specifically requested "No Gifts" on the invitation.

This also makes a great distraction, should you need one.  For example, if there is a lull in the party, problems with the stereo, or someone breaks a vase.  Simply, bring the guest of honor their gifts and a chair, while you go tend to your "emergency". 

Cake & candles:  For a birthday cocktail party, instead of making or buying a cake, use hors d'oeuvres or special bite-size sweets, such as petit fours. A fun option is to serve Jell-O shots.  Use unique candles in each one.

Unless you have a musical accompaniment or something unique planned, singing "Happy Birthday" is not recommended. This is all enough of a fun little interruption without forcing your guests to sing.
 
Cheers & toast:  Follow-up with a cheers to the honoree and get the party back on track.

Two words about balloons:  Overdone & unoriginal. Only use balloons in less traditional ways.  For example, tied to a float in the pool (or a dog), covering the ceiling (no strings), or tied end to end to reach 100 feet into the sky.  Otherwise, skip the balloons.