Probably a little too late to answer, but I'll do i anyway.
To avoid contamination we buy our TE-buffer, sterile water and sterile tubes. And if we have to make reagents or dilute our samples, we always use bought stuff free of DNA, DNases and RNases.
But the most important thing is to make sure that you work in a way that protects your samples and reagents from contamination. Work systematically, think about how you use your gloves (should you change them while your working?), think about were and how you treat the samples and reagents (is there a possibilty that you contaminate the reagents while you're working?), is your workstation and pipettes clean?, and so on, and so on... Routines while working are underestimated!
When we have had positive negative controls, it was always because we had contaminated our primers or something like that.