Experienced mom of 3 talking here:
We have travelled a lot through the years. First of all, I swear by our portable white noise machine. Turn it on and the kids know it's sleepy time, plus it drowns out all the weird noises you get in a hotel room. Get one now and start using every night and during nap time so they get used to it.
Also, bring a bottle of Pediacare or something else that knocks them out. This is for emergencies only---when it's 2 am and nothing seems to be working (or you're travelling cross country on an airplane during the crying to no avail stage). My pediatrician recommended this to me when my firstborn was little, so don't feel guilty!
Finally, make sure you bring their favorite blankies or whatever else they are used to sleeping with. But you probably already know this...