A stunning interview...advice needed.

I have an interview next week for a job I REALLY REALLY want.
It is for a really dynamic sounding not-for-profit and is for a job I am realisticly probably underqualified for, but it's in an area I have gained some experience in lately which is still quite new, so there's a good chance I am at least as knowledgable as any other applicants. The pay scale for the job STARTS at 10k more than I'm on now and goes up to almost double what I'm on now.
Anyway, I used to be super confident in interviews and actually got a couple of jobs based on my 'spunk', but lately I have been having a bad run in the job hunting and have had interview feedback that isn't so positive. I have been working in an assistant capacity and have lost some of my confidence.
I had a good interview at a recruitment agent the other day and felt like I acurately represented myself.
I have been doing some more reading round the area that the job is in so I feel confident in my kowledge, will thoroughly check out the organisation but would love any further advice on how to really make the job mine at the interview.
They also want me to do some psychometric testing, I have done this before for a job but only at the point of offering the job, not pre-interview. Is it best to be honest with these things or to say what they want to hear? I know if I'm honest it won't go down well as my personality is not the type usually coveted by companies. A nfp might value different traits though?

All advice gratefully received. Thanks in advance.

Lately I have not felt great about my interview skills but that may be because I've only had about 1 interview a year so I don't get much practice.

Just wanted to say that I believe you should tell them what they want to hear during the psychometric testing if that's possible. For example, if you once took home a stapler home from work, or took a puff off any illicit substance once when you were in college, it's best to tell them what they want to hear, and say 'No' instead of sticking to the honest truth. As far as deeper personality traits go I don't think it is possible to control the results.

Also, remember that you are worth the money they're paying. Even if you think you are under-qualified you do in fact probably have skills that they will find valuable.

Talked to someone in recruitment a party in the weekend and she said:
"If you have got the interview then you have the skills they want, now it's all up to you!"
I am determined to go and claim this job on Thursday, will let you know how it goes, because, quite frankly, if I do get it I wouldn't be able to shut up anyway!!!

Good luck with your own job search, and feel free to ask any advice if needed.