The only way is if you are a Paragon, Emerald, and to a lesser degree, Gold, and the plane is completely full in economy, and I mean *completely* full, w/o even one empty seat left. In order to get more economy passengers on the plane, they will upgrade as many of the elite members as they can.

That is the only way to get a complimentary upgrade on CI, and only happens once in about every 15-20 flights.

How you dress, or if you know the ground staff and/or cabin crew does not help at all. There is an elaborate system of checks and balances to make sure that no complimentary upgrades are given except in the case above.

If you are willing to use your miles, then it's pretty painless to get an upgrade as long as there is space in the cabin you are upgrading to.