Dress pants are fine if you are a man because they approximate the kind of pants you wear when competing. If you compete in Standard or Smooth, you could also wear a shirt and tie. Otherwise, any kind of shirt will do.
If you are female, then rather than wear pants, wear a skirt that is like the kind of skirt you will be competing in. So, that's a short flippy skirt for Latin, and a long flowy one for Standard or Smooth. Wear a top that won't show your stomach when you raise your arms. On your legs, wear whatever you would be competing in: bare legs, microfiber tights, fishnets, or nylons.
I see lots of people, including some pro men, practice in jeans. Jeans usually have some spandex in them these days, so if they fit right they're not too restricting. Personally, I can't stand to wear them, but that's just me.
If you're not a competitor, then just wear things that approximate what you like to wear when you go social dancing.