My philosophy on getting upgrades is to be courteous and pleasant - sometimes engaging the person at the front desk in light hearted conversation. I'm also in my 20s and travel for business on a weekly basis. When I check in, I'm in jeans (99% of the time ) because I like to be comfortable when I travel. I also fly first class in jeans and have not had a problem with the level of service I receive.

The key is to be a decent human being - not because you want an upgrade or something from them - but simply because you would expect the same out of them. With that in mind, you'll be surprised by the perks you receive.

As far as dress, hygiene is key. You'll always leave a better impression if you are clean and presentable in jeans as opposed to someone in a 3-piece suit who smells like rotten cheese.