Keep your instruments, fermentation vessel and everything else you are going to touch your must with clean and sanitized. There is no such thing as sterile meadmaking that I know of. It's kind of hard to fit everything into a giant autoclave, not to mention that it's pretty much a moot point since your yeast are designed to multiply so rapidly that they swarm and overrun any spoilage microorganisms present in the must. The best defense against infections in your mead is to follow proper aseptic technique, and make sure you practice your methods consistantly. I use four sets of egg timers when I'm making mead to ensure proper contact time with sanitizer, proper rehydration time for the yeast, proper atemperation time when I have to step down my yeast to must temperature.
As soon as you expose your nutrient to air, water, honey etc you are exposing it to airborne contaminants. Manufacturers do a pretty good job of making sure that they are not selling you infected ingredients because they wouldn't be in business long if they were sloppy, and they do have certain standards that they have to follow in order to sell their products. What those are, I don't know, but they're there nonetheless.
Cheers,
Oskaar