timppu: I keep hearing that normal dishwashing liquid is a definite no-no for cars. I don't remember the details, maybe it removes the wax too or something, making the paintjob dim fast.
I'd still suggest using car shampoos intended specifically for cars.
Ubivis: Never heard of it. Learned it from my father who is doing it for more than 30 years now. And his car is looking the best in the whole neighbourhood :)
First hand, seriously, I messed up my Benz with washing up liquid and ended up getting the whole thing resprayed. For two years I wondered why the wax wouldn't stay and wasn't protecting the car, didn't even think it could be the liquid. Sure enough, I've stopped using it, and the issues with the wax have stopped too.