Expressing sacs?

I just answered a question about this which is ironic. I knew my dog needed to have her sacs expressed because she was scooting and licking herself on the behind a lot. I immediately took her to the vet and it was only $15.00! This was the first time and she is almost 3 years old. I also feed her a vet recommended diet and she is not overweight. Anyway, I just wanted to know if this will always be something I will have to deal with or if this will be something that doesn't have to occur more than once or twice. I know the signs and will take her in again if it happens. Those of you with exprience with this, could you please let me know how often it's happened to you and if I should expect it to keep happening? I didn't think to ask the vet and the tech brought her back while the vet moved on the the next client.