Watch the Open Strings: Study 1 online guitar lesson by Jason Loughlin from Country Guitar Survival Guide: Lead Edition
This technique sure is purdy! Combining fretted notes along with open strings is a great way to beef up any simple melody by creating bell-like tones and adding more complexity to the harmony. This technique is known by a few different names such as open string licks or cascading licks. There are two components to this concept. First, use the interval of a second whenever possible. You do this by playing the next note of a scale on an adjacent string and letting the two notes ring into each other. The other is to substitute any note of a scale with an open string whenever possible. The more ringing the better. The sound created by doing this technique is similar to playing a scale on piano with the sustain pedal down. You get a lot of beautiful dissonance and overtones by letting each note ring into the next. Chet Atkins, Merle Travis, Jerry Reed, Albert Lee and many others use this technique. Check out Brent Mason's tune "Hot Wired," the head of the tune is built off of open string licks.
In this first instrumental study I'll be using major and mixolydian scales with some basic approach notes to create a bunch of open string licks. I am using hybrid picking here although it isn't necessary. Most of the time I'm picking fretted notes and plucking open strings with my middle finger. Try to get everything to ring into each other. It can be difficult to keep the pads of your fingers from muting adjacent strings.