Wouldn't exactly call myself a virtuoso, but maybe this will help: Although everybodies technique varies slightly, I have learned that there are 2 types of slappers out there....the "Flea's" and the "Claypools".
Claypool style is most common, which is slapping with your thumb parallel to the desired string and your wrist being mostly below the string you're hitting.
Flea style (the way I do it) is more of the neanderthal way of going about it. Its done by keeping your wrist above at an upward angle with your thumb going more across the string.
Both are equally affective, however getting a meaty, noticeable tone out of your D and G strings will have more to do with your tone then your actual technique. I haven't the foggiest idea as to what your rig is, but try boosting your midrange a little and cutting back on your treble just a tad. Also, try moving your contact point toward the pickups a little more. And be patient with it. The only real trick that I know to slapping lighter strings and still getting a full tone is by using the meat of your thumb a little more, which will help give it a little more thud. Remember, the string is smaller so dont be afraid to compensate your lack of presence by slapping it a little harder. Just make sure you control the strenght of your pop, or you will be breaking G strings like an overweight stripper. Hope I could help.
