First off, click
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If you are just starting out, make sure you take your time. Start with the basics such as Root/Octave passages (slap the root/pop the octave.) Try to keep the dynamics between slapping and popping equal, also. A softer attack will give a more consistant and well defined slap tone.
A book that I find to be very useful for this technique is "Slap It!," by Tony Oppenheim.