You should have up to date equipment/spells/etc. If you have a subjob, it should be *fully* levelled. (i.e., for a level 10 war, your sj should be level 5, in which case I will assume that when you ding 12, your sj will ding to 6, or you will leave to level it)

If no subjob, there's nothing you can do -- you'll be passed over for someone who has one.

Also, you should have an appropriate subjob. For warrior at low levels, this means monk. For whm, it should be blm or smn, and for blm it should be whm and nothing else. Warrior should never sub whm. Rdm should rarely or never be subbed. I will always pass over a /rdm sub for something more appropriate (I'd rather have a drk/blm than a drk/rdm, although for preference, drk should have war or thf subbed)

Two things that I would never invite into an exp party are people who have drg or brd subbed. Drg and brd are main jobs. Without the 2hr, drg is gimped, meaning that drg is always gimped as a sub. For bard, you need to have an instrument equipped to be effective, and you can only equip an instrument with a brd main. About the only time I'd consider brd as an acceptable sub is if you're a bst subbing it for the chr bonus--but then you'd be soloing anyways. Bst are OK for parties, as long as they don't charm, and you only need chr for charming.

At lower levels, I also tend to invite someone who's close--so if you're in the teens, put up your seek flag and stay in the Dunes. At higher (post-Kazham) levels, I'll tend to invite someone who is either in Jeuno or who has a search comment indicating that their HP is in Jeuno so they can Warp/deathwarp back in a jiffy. Jeuno's the gateway to the wider world.

And speaking of search comments--use one! A search comment can tell more about you than just your job, such as exp to next level (important if you might become too high for the party) I usually have a format like such:

@10000 tnl / HP: {Jeuno}
{English} / {Japanese} {Party} OK
(Any additional info I wish to add at the time, such as stat boosts, food type, or occasionally "nihongo de jouzu ja arimasen")