Repeating sidelines until someone (from Varsity) calls "last time" is exactly what we do. It took a bit of time for the girls to get used to doing it that way and to know how long to it keep going. We've been doing it that way for at least 2-3 years and they still give up sometimes before they should (especially when the crowd wants to yell "de-fense xx" on important 3rd downs

) I figure it takes about 1-2 times through a sideline to get the crowd's attention and then another time through for them to actually start yelling with you... even simple things like the school colors or letters, and of course they're more likely to do so if the game is competitive.
This year we are focusing on leading the crowd (rather than just entertaining them) during time-outs. Last year I had a few girls that were really good at feeling the energy of the crowd and doing what was appropriate for the moment: close game, lots of energy = crowd cheer, while in a blow out situtation when the crowd is sort of bored = more on the entertaining side (stunts, etc). Alas my really crowd savvy girls graduated.