You want to wash the shirts with laundry detergent and dry as normal (making sure not to use fabric softner or dryer sheets). It is important that you use the detergent to remove any and all sizing from the shirt before applying iron on.
As far as the creasing afterwards, I think that is more dependent on the transfers you are using than the care you do. The only thing special I do to mine is try to remember to turn them inside out before washing. But other than that, I wash and dry as I would my regular laundry. It really depends on the transfer and how well they wash. I have used some that ruined after the very first wash and others that have lasted over a year.
If you haven't already purchased your transfers, check out my webpage (link below) for the best transfers on the market.
If you have already bought your transfers, I would suggest that you not wash them again until after your trip - just in case - so they will look fabulous for your trip.