What do I do about my cat's dirty ?

This is quite a gross question, but I'm at my wit's end. My female cat has never been very good at cleaning her rear end. The last couple weeks her problem has become unbearable. Recently, due to the allergies of my other cat, I had to switch cat food brands. I'm worried that this might be what is causing her problem to become exacerbated. I can only get after her with a paper towel so often. She's a very affectionate cat, but I can't stand her to be near me because of the smell. Also, she's leaving marks on the carpet and furniture. I'm not sure if she's not getting enough oil in her diet, because she refuses to eat wet food, or oil from tuna, etc. PLEASE HELP!