Wedding gowns are tougher than you think. I used to design and make them for a living, and I was employed at Priscilla's of Boston as a alterations and finishing person.
Get lots of tissue paper -- the kind that has no bleach in it so it's archival quality, you can get it at most craft stores, it's more expensive but it'll be worth it. (Non-archival quality tissue can leave permanent spots on your gown. Never store textiles in paper that isn't archival quality, or if you use a sheet, make sure it's 100% cotton that hasn't been bleached and is well rinsed of soap.) Lay the gown front down on a bed or other clean surface. Lightly cover the gown completely with the tissue paper. If the bodice is NOT boned, start at the top of the bodice and roll the gown and tissue together in a not too tight not too loose roll down to the hem. Cover the roll with more tissue paper.
If the bodice IS boned, roll the bodice up from side to side, bundling up the sleeves along with it. Then roll up the skirt from the hem and carefully wrap it around the bodice. Cover the whole thing with more tissue paper.
Put the whole roll into a tote or duffle bag that won't allow it to come unrolled but doesn't squish it unduly. If it fits, put that into your suitcase.
If you have more room, stop by a dry cleaners or where you bought the gown and ask for a bodice form. This is an archival quality form of cardboardy stuff that you can put in the bodice of the gown. Assemble the form. Cover it with the tissue paper and place it inside the bodice, with the dress lying face down. Cover the bodice and form with more tissue paper, layer some tissue paper inside the skirt, and then fold the skirt up in sections the same height as the bodice form. Cover it all with more tissue and put it in an archival quality box, then put that into your large suitcase.
Basically the tissue is to keep most creases from forming permanent, so put tissue wherever that seems like it might be an issue.
When you arrive, the easiest thing to do is hire a finisher at a tailor's or seamstresses or bridal shop to press and steam the gown back into shape for you, although if you did the rolling properly, you won't have too many creases. Otherwise, do it yourself -- first press the bodice (you'll have to see if you can iron it directly or if you must use a pressing cloth or, if it's of lace, you'll need to steam it, and make sure you use a ham for the curved seams), then hang the gown properly (you may need to rig up a rope and chain to hang it so the skirt will easily drape over the ironing board), and iron the skirt. If there's an internal petticoat of tulle, press that first, starting at the top of each layer, inside out. The train is ironed last.
Let me know if you have questions! HTH!
