If you want the low down on degreaser for paint use, it's called P21s total auto wash. It's around $70 for a bit over a gallon, but it's specifically designed for paint. It has some washing properties as well as an incredible cleaning ability. You can use it straight or dillute it some. I primarily use it for engines and lower body grime. Very potent, so you don't need much. It'll strip the wax too, so it's more of a treatment done before a full detail, than as a regular maintenance thing. I think for much less money, you could probably mix some regular citrus degreaser with some water and car wash soap and come up with something reasonably effective. I'd keep it in a spray bottle, since you won't need much. With the soap in it, it shouldn't etch the paint or anything, just work somewhat quickly.
