Masks are up there with cuffs: they're pretty much part of the ABCs for bondage. Masks come in all different types. Some have mouth coverings, some have open mouths. Some have removable blind folds, some have padded blindfolds, some have attached blindfolds. Some are made of leather, some made of rubber, some made of latex.
We have two masks and we use one in almost every scenario. The first is a spandex mask. It's stretchy, it's easy to pull over his head. The eyes are covered with a padded area, making it basically impossible for him to see out, but the mouth and nose are just covered in spandex, so he can breathe. I like it because (a) it's very easy-on, easy-off and (b) it allows me access to his mouth. This lets me kiss him when I feel like it. I just roll up the mask and then roll it back down. It also allows him to make semi-articulate noises, which is important if we're trying something new and I need feedback as we go along.
The other mask we use is more of a hard core device, but X likes it better. It's leather. It has a removable blindfold that secures with snaps. It has a removable gag. It secures to the back of the head with several straps that can be buckled at different tightnesses to keep it snug. X likes it better because he really likes the gag. One issue I have with it is that because it's firm leather, it's difficult to get situated on his head. What often happens is that I tell him to put it on and then I secure the gag, thus showing who's boss but still letting him get it on so it's easier on me.
If you're not sure if you want to try a mask, try blindfolding your partner first and see if he likes that. You can use thick ribbon, knee-high socks, or a large stocking hat (although that could get hot). You could also just throw a shirt over his head. The important thing is that his eyes are covered; how you get there is up to you.
I mentioned gags, but there will be a separate post on them. There's a trick to them.