Having your wedding dress altered is a necessary part of wedding planning, and it is quite an undertaking in itself. Wedding and formal gowns are not made from the fabrics that we are used to wearing everyday. These fabrics are not typically "stretchy" so they need to be altered to fit your shape. All gowns are structured differently and each woman has her own contours that are unique to her, the key is to mold the dress to your body shape while keeping the integrity of the gown design. The good news is that most formal gowns are made with alterations in mind, the bad news; this takes time and multiple fittings.
The most common misconception that I see with alterations is that people don't understand why it takes more than one fitting, or why the dress is not perfect at the second fitting. Depending on the dress, the person, and the type of alteration it can take several fittings to get a dress just right.
"Show up with an open mind. It may take more than one appointment to get a perfect fit, so don't despair if the dress puckers or pinches." Randy Fenoli - Much Ado About Alterations
If you are in the alteration process, please be patient with your seamstress, remember she/he is trying to mold this delicate, unforgiving fabric to the shape of your body, and that is not an easy task. Everything that is done to one part of dress can affect another part, which it is not always predictable.
For more information on this subject please read; Much Ado About Alterations, a must read article about the alteration process based on an interview with Randy Fenoli of Say Yes to the Dress.