Ok zaccy I have done this many times, here is how it works.
First you need to scour the internet for the best price you can find for what you are looking for. Personally I would suggest buying all components separately, as in airfares, accom, transfers etc, as I think you can get the better deal that way as opposed to buying an entire package.
Once you have found the best price simply print it off and take it in to flight centre. They will tell you what price they can do for the same thing. If the price you have is less - and it usually will be - then just ask them to match the printed quote you have given them. They will then match the price and give you a further $1 off each item. Then they should give you a $20 FC voucher for each person.
The $20 voucher can not be used in that same transaction, it can only be used on something else. So if you want to buy tickets or anything else just get them at a later date and you can use the vouchers then.
The only thing to really be careful of is that they will only match prices from Australian websites. So the airlines websites are fine, they will match them and also Expedia.com.au, but not Expedia.com. I have found Expedia to normally have really great prices, especially on park hoppers.
Remember zaccy not to be shy. Ask for what you are entitled to. FC are a multi-million dollar corporation who will take you for a ride given the opportunity. Walk in there like you know what you are talking about and talk to them like you have done your research. They are the ones that advertise price beating so don't be embarrassed to ask for it.
Hope that helps and good luck
xxx