Buy good equipment (good =/= perfect). I won't say there's no skill in XI, but there's little that skill can do if you don't have the equipment to carry it out. IE: skill arguably means you never miss a skillchain, but equipment means you'll actually have tp and your weaponskill will actually deal damage. Never missing a skillchain is great- never having TP and producing crappy damage is bad. Part of skill is knowing what gear you need to do your job.

Use search comments. People who read search comments tend to make better parties than people who don't. You'll be increasing your odds of being invited to a party with a knowledgable leader this way. Leaders who read search comments often check zones for overcrowdedness and pick the right jobs for the task as well.

Carry silent oil and prism powder. Noone likes the reject who can't even get to camp safely w/o help. Often, a mage will save everyone some gil by doing a free first casting of sneak and invis on entering a zone, but after that you'll be on your own. Dying enroute to camp is a no-no.

Keep your /party macros to a minimum. Roleplaying is cool, but a macroed tagline on your commonly used abilities will irritate people. Dark Knights are especially prone to this. Noone cares how dark and brooding you are - just report your TP like a good melee and make the SC.