Here's an easy tactic to build some Happy Hour business and it works time and time again. Drop the fishbowl at the door with signage promoting giving away a happy hour party each week. Then, on Monday, pull all the cards and email out a congratulatory email, letting them know they have won a happy hour party for the office this Friday.
Also, in addition to just dropping the fishbowl, encourage staff to ask guests for cards so they can enter them as well.