As a professional bridesmaid and wedding go-er, here's your pro tip: never get your dress altered at the store you purchased it. It's always going to be way higher. Sorry David's Bridal.
My most recent trip here was for my Vera Wang bridesmaid dress. It was way too long. They shortened it for $40. A friend in the same wedding with the same dress had hers both shortened and taken in as well. She spent around $50.
Process is simple. Bring your clothes in, put them on in one of the dressing rooms, step out and onto a block. One of the nice ladies will evaluate and talk you through what you want. Put the pins in and mark where the changes need to be made. Usually takes about a week to get it done.
I don't get clothes tailored or altered a whole lot, but when I do this is my place. Thanks ladies of Anna's for all you do.