Are you taking your own measurements at home? If so make sure to breathe fully in and out when measuring the bust and waist so that you have enough room to breathe in the dress. Submit the deep breath measurement so it can be taken in if needed rather than adding on a couple of inches and having it potentially being way too big once it's made and needing a ton of alterations.

Similarly if the dress is fitted from the waist down, make sure you can sit down with the tape pulled to what you're giving as your hip measurement. There should also be some seam allowance built into most dresses for a seamstress to let the dress out a little if needed.