In OB, if you get some crooked sits, like me, then black pants are best if you have a black dog. In OB, don't wear skirts, but most dress up a little, like slacks or nice jeans. I've never done Agility but I think jeans or shorts are fine, whatever you can run in.
I generally wear black slacks or Khakis for obedience/rally. I've started wearing black slacks more for the reason someone else gave . I have some black sneakers also. I know some people wear jeans in the obedience ring, but I don't. It is a show and I just think that jeans don't quite cut it. I can see them more in agility where people are running their butts off - LOL!
For a match show, I generally would wear just khakis, but lots of people wear jeans. I probably will with my next puppy. I'm pretty much a jeans and t-shirt kind of person.
Very casual is fine for matches of any sort--conformation, obedience, rally.
For obedience the latest obedience protocol says you wear black slacks or skirt (frankly it used to be that only trainers (as in profecssional types) wore that particular uniform in the ring but these days it's supposed to be polite to look a little dressy--they won't likely get me in any obedience ring in anything except jeans and probably t-shirts. Rally is much the same.
For agility if you're lucky and your dog hasn't knocked you down in the ring because you ran into each other your clothes (jeans, shorts, whatever you can run in and is comfortable for you) are still intact. Sometimes you aren't even that lucky.
Anyone want to guess what you wear tracking? LOL!!!! That's always a good laugh--usually waterproof boots of some sort if it's a spring or fall trial. Full blown foul weather gear if it looks like rain. As little as possible if it's a summer trial (thank heavens there are many of those)--and you just hope that the track layers didn't make you go through any blackberries.