Never used a mr. beer, but other than sanitation, I would say to make sure you keep the temperature in the correct range for the yeast. I don't know what you're making, but if it's an ale, I would say try to hold the fermentation temp in the low 60's. Don't keep it on the counter in your house if the temp is 75. Go for a cool spot in a basement, closet, etc. to make sure you ferment in the right range. You can even keep the fermenter in a larger water bath (bucket) to minimize the temperature swings. No need to add ice if the temp is already in that range.
Needless to say, temp control is important for making beer. I didn't realize how off flavored my beer was until I actually controlled temperature.