I have a completely open schedule. I started as a cashier. Even when they were cutting cashier hours last year, I almost always worked full time hours.. Not because I was scheduled for those hours but because I constantly got called in to cover shifts when people called out.
Key to getting those calls is to almost always saying yes and work your butt off when you are there.. Don't complain, unless in a joking manner. Be personable to TM's, tl's, etl's and guests.. Do what you are asked to do. Know your job, never be idle. Don't boast that you have a full schedule (those who don't will not like you) and be willing to do what it takes to make your area look good. And of course, no matter what department you work in, get redcards..they are like gold to target....
There are no guarantees that you will get the extra hours. If you are looking for a job with a definite number of hours, you may want to look outside retail....