I have to give credit where it's due... I have been on the hunt for the PERFECT bootie for a long time. Something quick, minimal sewing, cute, doesn't fall off, and no seams showing. Well, I found it on the <"a href="http://dorothy-comewhatmayandloveit.blogspot.ca/2010/01/bootie-tutorial.html">Dorothy Was Here blog. Her slippers/booties are so amazingly wonderful that there are no words to describe them. I have made several pairs (even some for donation) and upsizing is not hard to eyeball.
Naturally, if you're going to make a bajillion, or if they're not for home use, please head over to her blog and see her rules on that.