+1 on the foam stopper. I dunk mine in StarSan water squeeze it out really well and plug it in the flask.
I also wanted to add my two cents on krausen formation on a starter. I've seen a bunch of people who mention getting little or no krausen forming. I've made 4 different ones now, with 3 different kinds of yeast - I have yet to see any significant krausen formation. Just some CO2 bubbles in suspension and a layer of yeast forming up on the bottom.
Also, I saw someone mention before mention this, but be careful swirling! I had an explosion on my first starter when I picked it up and swirled it too vigorously. What a sticky mess!
BTW, I've been following the starter recipe from Northern Brewer: