I find that if I strike the string by rotating my fore-arm (kinda like tunrning a door-knob) I get more accuracy than moving my whole arm back and forth.
Are you slapping with your thumb pointing up (Victor Wooten, for example) or pointing slightly down (Flea, for example)? In my opinion, the "traditional" way (thumb pointing up) allows for more efficiency. As was mentioned before, practice slowly (whether you want to play "through the string" as was suggested is up to you). It's largely about building up muscle memory and, generally speaking, the D and G strings are harder to nail with consistency than the E and the A strings. Practice striking each string slowly - from E to G and then back up. Then try playing through some scales (again, nice and slowly). It can be frustrating at the beginning, as you are likely to also encounter some problems with unwanted noise from the other strings. Just be patient and persevere - you'll get it.
I could recommend a good book that takes you from the very basics up to the more advanced stuff - or so I'm told, I'm still stuck at the basics Since I'm new here I don't know what is considered "spam" I did not write the book myself, nor have any interest in it (i.e. do not get a percentage of sales). PM me and I'll tell you what it is
Since I'm new here I don't know what is considered "spam" 
I did not write the book myself, nor have any interest in it (i.e. do not get a percentage of sales). PM me and I'll tell you what it is 
