Hi,
This is my first post and I hope that my experience in slapping would benefit you.
I have just learned how to slap and pop last month. Just started my bass guitar lessons and the teacher has been really great.
I was doing my own kind of slap before I had the lessons where my thumb was parellel to the strings and I hit the strings with the thumb joint.
According to my teacher, that is kind of new skool. So he thought me the old school ways so that I can choose which one I am more comfortable with.
Finally, I have decided to go with the old skool technique of slapping.
OLD SKOOL SLAPPING TECHNIQUE
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1. Thumb points to your left shoulder ( If you pluck with your right hand)
2. Make sure yout thumb is straight and bend it over to the back as much as possible (if you are not double jointed, it will be harder but it's not a must. Just make sure your thumb is straighten)
3. I choose to slap the end part of my bass neckboard cause it sounds more full there. When you attack the string with your straighten thumb, make sure it points to your left shoulder at all times.
4. Once you slap the string, quickly lift it back up so you can hear the sustained note. It's more of a wrist action.
5. When I started slapping, I was hitting both the A string when I slapped the E string and vice versa. It's annoying but after persistent practises, i am getting more precision. Some people will tend to overlook this and concentrate on the fretting hand to deaden the unwanted sound from the other strings. But i have chosen precision instead because it helps a lot if you want to learn harder slap pop combos.
6. Some people choose to slap through the strings instead of bouncing back up after a slap. I slap through only on the D and G strings to get a clearer tone. Instead on lifting your thumb after hitting the strings, you kind of follow through to the string below (not necessarily touching the string below)
7. Finally, for precision when beginning how to slap the E and A strings, the best thing is to sit in front of the telly and continuously slap on the E or A strings. You can mute it with your fretting hands or do it on open strings, just keep doing it till you don't have to look at the thumb. Within a week, you will realise that it's like second nature.
Have fun! I know I did.
And finally, check out this website if you are still in doubt I I was and it helped me quite a lot)
www.gazzbass.com
and
www.warwickbass.com
Andre
slapping technique 
Heh, not everyone has access to a fretless though.
