Maybe it's just my dry cleaner at home, but I've found the ship prices to be lower most of the time for cleaning/pressing my formal gown. I bring it all wrinkled and stuffed into my suitcase. The minute I get my bags I take it out and give it to the steward/ess and send it for cleaning. Most of the time I have it back the next day for the first formal evening. It's clean and perfectly pressed. It might be more expensive to have other things dry cleaned or pressed, but for gowns I think it's very reasonable.
I see a lot of people saying they use some kind of a spray wrinkle release product that works well. I keep forgetting the name though. But then they say there are three things that go when you get older - memory, hair, and I can't recall the third one .