When you get to the changing room, all dresses should be taken out of their garment bags and hung up. Some fabrics will melt if they're ironed, and many embellishments shouldn't be ironed or steamed, so get care instructions from the store or alternations staff when you pick up the wedding dress and bridesmaids' dresses.
If the bridesmaids' dresses are looking wrinkled or flat, hang them in the bathroom and run a hot shower for ten minutes with the bathroom door closed; turn off the shower and leave the dresses hanging for another fifteen minutes. The steam should get rid of most of the wrinkles. Next, take a clean white towel and gently rub it down the length of the dresses to smooth out the fabric. If there are still wrinkles, it's ok to touch up the dresses with a handheld steamer.
The wedding gown is higher maintenance, particular if it's white or ivory, and particularly if it's satin. If the bodice and sleeves were stuffed properly when you had your alternations done, it should have traveled fairly well. Hang it high enough so that the skirt doesn't touch the floor. Most closet rails aren't high enough to do the trick, but often the door of an armoire, a door frame, or a window frame will work. (If your dress is heavy, be cautious about hanging it from a curtain rod – you don't dress the rod, dusty curtains, and your dress to end up in a tangle on the floor.)
One of the biggest challenges with a white dress is keeping it pristine till the wedding. Inspect the area where you're going to hang the dress and wipe off any dirt or dust with a towel. Also lay a white towel on the floor underneath the dress to keep the hem clean. Hang up the veil at the same time; everyone tends to forget that veils wrinkle too.
Many higher-end hotels have laundry service staff experienced in steaming wedding dresses. Make arrangements in advance, though – experienced steamer don't work 24-7.
If there isn't a professional service around, apply the same steaming procedures described for the bridesmaids' dresses, taking care to put a white towel on the floor beneath the dress wherever you hang it.