God I hate to say this... Scales! ... You might want to just practice hitting a clean scale pattern with your lead finger in whatever major tuning you're in.

I'm finding I like the open G tuning that Bluegrassers use more than I like the double C (D if capoed up two) tuning that old-timey players seem to favor for fiddle tunes. If you need to play in D, just tune to an open A (a step above G) and then capo at the 5th fret.

I also sometimes sketch out the melody with my thumb before I try playing it with the lead finger. The thumb seems to "remember" better, and once it knows it teaches it to the other finger. I believe I saw one time that the thumb is controlled by more brain cels than any of the other digits.

And, like learning the basic frailing motion, it's practice, practice, practice! I'm starting to try to learn the 'drop thumb' lick now, and like everything else in this style, it's gonna take awhile.