I wear something like Dockers and a polo shirt, unless I'm speaking, then a shirt and tie.
But probably the most common outfit I've seen at EMS conferences involves jeans or EMS pants with a "tactical" nylon belt along with a T-shirt that says "ABSOLUTE EMT: My Job Is To SAVE Your ***, Not KISS It". Or some variation thereof.
Don't listen to any of those people; if you're such a high-speed low-drag EMT that you're going to a conference, you want all the people you meet to KNOW that. I mean, everybody knows the point of going is to show off what a kickass medical guru you are, not to learn anything. So make sure you were your uniform pants, boots, and a tacticool belt loaded with all those nifty gizmos; super-fancy shears, badass combination knife, window punch, mini-mag (LED of course), glove pouch, tourniquet (in tactical black obviously) and the most expensive stethoscope you can buy. Don't wear your uniform shirt though; you want people to have to guess where such a badass like you works; it'll make you that much more mysterious. What you want to wear is a nice "racing the reaper" type shirt or "doing what doctors do at 90mph" type t-shirt; that make everyone take notice and tells them exactly what you do. And the most important part; make sure you keep a nice big pager attached to your belt. Doesn't matter how far away from your service area you are or if you're off duty, you could be called into action at any minute, and you want EVERYONE to know it.
Or you could just wear jeans and a shirt...but wheres the fun in that?
I usually do jeans and a polo or khakis and a polo depending on wjat I'm doing at the conference. Went one day in ems pants and a tshirt but I came off shift to go to a class, so I figured I would be ok