Well, back when I COULD play such things, I would practice seven "positions" of major and minor scales (one "position for each note of the scale, so your first finger starts on the first note of each "mode," though this is NOT necessarily modal playing).
But the more interesting stuff came from practicing them in patterns other than seconds. Try other intervals, such as playing them in thirds, or fourths. My warm up used to be running through each key through the cycle of fifths, and in each new key I would play a different interval (starting in a different key every day so that I didn't only play the same key in the same interval over and over). Sevenths are fucking HARD, because if you want to stay strictly positional (and you should learn to do it that way, simply because it is good practice) you have to do a lot of two or three string jumps with your fingers, and doing it in time is TOUGH. (You do practice these things to a metronome, right? If not, you should.)
Also, go buy Mick Goodrick's book, "The Advancing Guitarist." It is not a method book so much as a discussion of how to think, and it is VERY useful in that regard. He spends a fair bit of time talking about single strings scales, and you can get a lot out of them. They are not going to build amazing chops, but it will help you learn the fingerboard, and will help you be a player who thinks in terms of NOTES and not PATTERNS, and thinking in terms of notes is the path to truly creative and original music, as opposed to just playing licks. Mick Goodrick is one hell of a player, by the way. This is a guy who has never had a driver's license, because he doesn't want to use any part of his mind that could go to guitar. He has a lot of valuable ideas. I used to have a teacher who studied with Goodrick, and my teacher said Goodrick was one of the best teachers ever.
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