That's the big question, isn't it! I don't know of any techniques in the formal sense but I do have some thoughts that might help. I really don't know what level you are so I may not be of much help - I'm just thinking out loud here.
For starters, they play the melody and then play around the melody. Doc cut his teeth on fiddle tunes and knows the melody to tons of them. Even if you don't know the melody of a specific tune, if you have a large arsenal of melodies at your disposal you can cut and paste with nearly endless possibilities.
Thinking in terms of a melodic phrase (try to ask a question and then answer it) can do a lot to keep things fresh.
When you learn a lick, learn to play around with it - tony can be very licky but he also knows how to mess with it. The trick is to not have a lick set in stone but rather to know what the lick sounds like, learn what parts of the lick give which sound, and then practice rearranging it.
Really utilize different parts of the neck and find ways to transition between them. This isn't AS big in bluegrass as it is with other styles (I swear, we capo everything for the TONE! ) but it's still pretty big and it can add a whole dimension to your playing.
Listen to the guys Tony and Doc listened to. Find out their inspirations!
Try to play like a fiddle player - if I have my history right, Doc started flat picking to fill in for a fiddle player at a dance. From the get-go he was trying to sound like a fiddle player.
Play the G run ALL the time!!! Just kidding ;)
Really though it's all about melodic playing. Great players let the song speak for itself and try not to get in the way of the music.
Also are you up on Norman Blake?