There are 2 ways of dealing with nebenkosten: akonto or pauschal. Akonto, as pointed out, means a fixed amount is paid per year, then at the end of the year this is compared to the actual costs and the tenant either get some monies erfunded or has to pay the difference. Pauschal is also optional, but is getting less common - this is a fixed cost per month, without any extra to pay or get back at the end of the year. The actual pauschal is calculated every 3 years or so.
So... check your contract to see if you're on an 'akonto' or 'pauschal' payment option.
For extra info about nebenkosten, there is quite a good website at
http://www.heizabrechnung.ch/ but it is all in German.