Without giving specific numbers, be able to show you've done your research on both the sector and whatvitvoays generally and what the job may pay in particular.
For example: "I know the market rate for this job is higher/lower than what you're currently advertising. However, as this is a charity/the economy is putting pressure on wages/this job will require considerable overtime (find your valid reason) so I am prepared to work for below market rate/the salary you advertised is something I can accept." Remember the answer is about demonstrating you know your sector and have done your research so if they offer it to you no one will be surprised that either sides salary expectations are that out of synch.
Do not, under any circumstances, do what my friend just did in the US. She applied for a job, not in her field, starting over and got an interview. The night before the interview, we were IM-ing and she tells me about it and that she hasn't worked out if she can afford to take the job if offered. The salary range offered was $30k - $40k. She gave me a number for rent (shes currently living with her parents but wants to move back to the city and would have to for this job) and for child care. I multiplied those by 12 and it came to $42k.
She genuinely then wondered if she could get $42k for the job because that's what her costs would be.
- she forgot about deductions for tax, etc, not to mention eating, petrol, cell phone bills, etc. so on her numbers she was going to need a LOT more than $42k (I think she also needed to find a way to reduce her numbers, but still).
- she had no experience in this field. You simply can't walk in and say "I can't take this job for less than over your top range" unless they are desperate to have you for some reason. It is about what they want to pay you, not what you need to live on. The more experience you have, etc the more they want to pay you to get you above someone else. At least that's how it is supposed to work - the short version.
I tried to be diplomatic in explaining this to her, far more so than I am being here. Maybe too much so - she went in and told them she would happily take the job but only for $55k.
Now she doesn't understand why they wouldn't give her the job or why her applications for other jobs in the organisation for the same pay range don't get her interviews.