I buy cute, inexpensive white bags... generally from Target, Macys or Dillards. I usually get a little more "trendy" with my white bags...not too much, though.. I'm over 40 and don't want to look silly. I spend no more than $50...use it for the summer then either put it in the Goodwill bag or give it to my 8 y/o daughter.
I am also afraid of dirt and "yellowing" so, for me...not spending a lot of money on a white bag is the best solution!