i think the person who wrote the paper may want to go first, because without them putting it together there would be no paper.
otherwise, do it alphabetically and it is presumed you are equal contributers.
i have had this problem with co-author's before. where i do all the writing and provide most of the data and they try to submit the paper with their name first. people are idiots! after am done with my two papers, it will be while before i co-author.