I found scales and exercises to be very helpful in developing slap techniques. Found I didn't get sucked into speeding like I did when I practised other peoples lines. Also you should learn from the start how to stop unwanted strings from ringing out. This will affect the angle you hold your hand. Higher pitched strings can be easily damped with you fretting hand. Lower pitched ones can be muted with your forearm or the heel of your hand. Very important if you have a B string. But if you learn from the start you won't have to relearn your technique when you get a 5 string.
After you can slap a scale up and down, exercises like C,E,D,F,E,G,F,A... or C,D,E,F,D,E,F,G,E,F,G,A etc, up and down 2 octave scales and around the cycle of 5ths will help with accurately changing strings as you slap. Bigger interval exercises, like 1,6,7,2,3,8,9,4,5 etc will get you precisely skipping over strings while avoiding the pain and potential damage of holding an octave shape for long periods. Alternatively, run 4 chromatic disco octaves then play a scale up or down to practise transitioning into and out of them while keeping the blood flowing in your fingers. Your hands will thank you when you're old.