While I am not any kind of example of excellent roleplay
I figure i am the missing link between the good rp people
and the newbies, so here is what I have on it.

Where straklaw talks about playing something akin to
your own personality this is a GOOD thing.
While many people disagree or look down on this, You don't
want to get too far from 'you' until you have a handle on it.

ex:
my first paladin (which i wasn't in any way ready to play)
had a role not too far from the blip cerunnir posted
but i played him as ultra sad and self effacing
(the self effacing part didn't go over too well with the
empowering imm nor did being very pro elf and not so pro other
races of the light)
The problem is that I am anything but sad. IRL i laugh at
everything. (too much acid in the 80's i guess)
So i couldn't keep it up. and it ruined the character.

so play something you are familiar with. I have played many characters
nothing at all like me. none of those ever hero, none get anywhere
etc... not sure why. but the ones i play that are like me seem to do
okay. I guess i have a likeable personality and people like to
group with me so i at least always have fun.
which is what it boils down to have fun.
If you are self-effacing like myself after you get a handle on
playing you THEN try playing the arrogant dark-elf healer who
thinks the world is his and the memo just didn't get sent.

(though being humble or down on yourself in any way seems to
be a very bad bad thing from imms and morts perpectives alike
so maybe not wise to play that part of your personality)

and hell while you are at it, if you are playing something
like yourself throw in some emotes that you do under certain circumstances :the thought will come natural if not the typing.
If you rub the back of your neck when you are lying (how my mom
always caught me) or if you run your hand through you hair when
you are thinking of what to say ... do it in game. That'll flesh
out a character fast.