The service unit manager and assistant managers dress up for their monthly meetings. For example, in November the manager and assistant managers might dress up in Thanksgiving themed costumes. The leaders enjoy this, and it doesn't take long for attendance to pick up because they are curious to see what might happen next month. Of course, it is impossible to dress up every meeting, but the leaders know if they keep coming the managers would certainly appear in costume again. In October, you might even have a costume meeting and have everyone come in costumes.
Award prizes for the best costumes.
This is an old but guaranteed show stopper. Scout Bucks, by far, is one of the most successful ways of assuring attendance at Service Unit meetings. Never announce the auction ahead of time, hints are fine, but don't let them know the exact meeting of the auction. Normally, you will have two big auctions but occasionally you can throw in a small auction with only one or two items for bid. Make sure your different denominations of money are printed in different colors.