I started washing mine with dish soap yesterday because I had nothing else in the bathroom (I have one of those foaming pump dispensers with some dish soap in it. I use it mainly for washing my glasses, [yes, the kind I wear on my face] but I'll use it to wash my hands if there's nothing else), and now the cup hasn't been leaking anymore.
The cup worked perfectly for me for the first 3 months or so, and then it started leaking constantly, no matter what I did. I used to wash it with Dove sensitive skin soap. I think that because Dove leaves a bit of a residue (the moisturizers in the soap), it might keep the cup from getting a good seal. I think that since dish soap gets it "squeaky clean", that might help the skin inside the vagina 'stick' to it better, and create a better seal.
I think maybe it worked for the first 3 months because it had no soap residue on it, and then when it got a bit of a buildup, it started leaking.
If anyone is curious, it's palmolive dish soap.
Edit: Well, so much for that theory. It's leaking again. Just when I think I have it figured out, the cup goes "PBLBLBLBLBLTT" and leaks on me.