* be polite
* smartly dress (like just dont wear the dunlop volleys & ripped up jeans - dont go all out with a suit lol)
* dont get drunk
* do some googling before you visit her. See if her dad/mum runs a company or something - how thats going? dont bring up work if they're having legal/financial issues etc.
* ask your gf if there are any controversial issues not to talk about (see next 2 points)
* dont say stupid shit - dont talk about sex, masturbating, drugs, thieft, porn, fucked up shit you've seen on the internet etc. (dont talk about something which you know will rub them up the wrong way,
* Dont talk about controversial issues that are personal to the family ( i.e. i nearly spoke about illegal downloads when the mum worked at MIPI), but could include things like adoption, suicide, divorce, termination, miscarrage etc. (some of these issues can be talked about, but me mature about it. i.e. dont say things like "aha my brother is a joke, he's adopted".)
* Be considerate of things like religion: if you're not sure about her religious standards google it (i.e. if there are certain thing you need to do at the dinner table).
* talk about your future (especially uni and things like that - if you're not that age, then subjects is okay.)
* relax, try and be yourself
* dont quote movies/tv as jokes which you know they probably havent seen - its okay to talk about a topic/theme etc, but don't go on about it - particular shows like southpark [describing mr hanky the christmas poo just wont work]
the gf will recognise if you're not being yourself, so try and be yourself
. (lol quite hard after previosuly mentioned list). If you use common sense then you should be okay
EDIT - one thing i think is important is giving the dad a good 'firm' handshake, look him in the eyes and introduce youself. It also might lighten the mood a bit, particularly if hes nervous about his "little girl" going out with guys.