AernaLingus' advice is good and I'll add just one more thing: testing proper fit. If you have a good seal and the mask is at all flexible, breathing in should suck the mask into your face while breathing out should push the mask away but not so much that it loses seal. Breathing in and out quickly makes this more obvious. If I do this with a mask that is not fit well there is no sucking in / resistance because air is coming in through the leaks. This is how I settle on masks for myself aside from checking for NIOSH / equivalent ratings.