Fly line has some impact on it; the Orvis Hydros Easy Mend for example picks up really nice off the water (that's largely what I use).
Anyway, regarding the technique, practice in a grassy area and work on flipping the rod tip almost like you were flipping a pancake. It's a short, methodical flip of the wrist. once you get the small ones down, increasing the effort in the flip will result in a larger mend.
Also, if you checkout the Orvis fly fishing website look for Pete Kutzer's videos on techniques, I'm sure he has one in there on mending.
Cheap answer: practice with the line you have (you'll get it).
Slightly more expensive: you still have to practice, but with a line that picks up off the water easier due to the dimpling in the line itself.
Hope that helps,
BW
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