One of the smartest things I did was prioritize comfort from the beginning of my dress shopping experience. Of course the most beautiful things are often the most uncomfortable, but there are plenty of items that bridge the gap between beauty and comfort. And since your wedding day should be about your enjoyment first and foremost, you better think about comfort.
Personally, I didn't think about comfort enough. Had I thought about it more, I would've tailored the dress so it wasn't so low cut (I did feel a little over-exposed). I also would've sewn little strings and snaps into the straps to secure my bra (like Ann Taylor does in their dresses).
For the most part, however, I was super-comfortable. It helped being able to wear normal undergarments.
A lot of different things go into minimizing stress and maximizing happiness on your wedding day. I'm convinced that being comfortable in your clothing is one of those things!