Well, my mom can be difficult as well. I buy what she needs, but does not realize she needs. This year, she is getting three pairs of lounge/velour kind of pants for around the house, three sweaters for work, two different body washes, some makeup, and a magazine subscription.
What about tickets for you and your mom to go to a play, concert, etc., or tickets for her and a friend to go, if you don't live close? If you went w/ her, take her to a nice dinner or if buy for her and a friend, get them a gift certificate to a nice restaurant near where they are going to make a night of it. Spa or salon gift certificates would likely be appreciated also, if your mom tends to buy whatever she needs/or is difficult to purchase for.