You gotta work it! Double slap is just hitting the string twice with your thumb. Sounds easy, but it's really tough for fast songs. Check out Red Hot Chili Peppers' "Get Up and Jump". That song has a cool double slap bass line.
All I can say is keep on practicing to building accuracy and speed. Also, there's a technique you can do to substitute for the sound of double slap. I don't know the name, but I'll run you through it:
Your fretting hand will use this form: Take your open palm, and bend your index finger down, so all your fingers are up except the index. You use this form on the neck - your hand wraps around the neck like it always does and the bent index finger holds down a note on the fretboard. Remember to keep your other fingers up.
Now pop the string your index is on. Then have your middle, ring, and pinky flap down on all the strings simutaneously. Don't press anything, but do something like a three-finger mute with a bit of more power to "slap" the strings.
Then slap a string with your thumb, your middle, ring, and pinky still down to mute the string you're slapping.
The resulting sound is like this:
[pop] [phat] [mute slap]
The "phat" is the sound of your three fingers flapping down on the strings.
Again, this requires practice to build speed and accuracy - especially accuracy. To learn this technique, check out www.gazzbass.com. The bassist there, Gary Dreyser, I believe, makes video tutorials and is particularly fond of Mark King's bass style.
You can see him do the technique I'm discussing early in the video.
Double slap and the technique I discussed both have their rhytmic qualities. Double slap tends to have more edge and "oomph" to it, while the other technique has a more clean, almost "jazz-oriented" sound where you can seriously build some speed.
Hope that helps! And a quote from my middle school orchestra teacher: "Practice, practice, practice!" Hahaha