I use a Mason jar, the kind for homemade jams, but instead of using the flat metal lid I use a piece of clean fabric held in place by the metal ring. You have to let that mother breathe.

reminds me of my initial mistake with my "mother"--too small a jar, foamed up and out and made a huge mess in the fridge. now i use a much larger mason jar, the starter takes up a little over half the volume when more dormant, and the screw-on lid is just gently tightened, plenty of escape room for the gases.

I use a Rubbermaid too. Like cookbookchick, I don't seal it either. I have kept mine in a mason jar too, but I prefer a wide-mouthed Rubbermaid as it makes it easier to scoop the starter out.

Ceramic/pottery jars work well that have lids. King Arthurs Flour sells one for this purpose but any canister like that will work. Make sure you give it some head space 1/3 starter 2/3 air space is a good rule of thumb.

I keep mine in a mason jar as well but sealed, I haven't had a problem. Maybe after a day in the fridge it will become dormant again and the gasses will subside and you will be able to seal it completely.

Don't seal the container -- just lay the top on it or cover it loosely with plastic wrap. I keep mine in an Oxo container, the kind with the pop-up in the center of the lid, and just leave the pop--up "up." I use one large enough so that there is a lot of air space between the starter and the top of the container. I also sometimes keep some starter in a glass mason jar with the cap on but not not screwed down. I'm sure others will have suggestions, as well. By the way, the starter will keep for quite a long while in the fridge until you are ready to feed it again.