What is a string skipping exercise? I've heard about this exercise a lot, including on this website. What is it? What skills does it develop? Can you give an example?
String skipping exercises could be anything from playing arpeggios by using notes on alternate strings to playing phrases using scale runs using alternate strings, using string skipping is essential to making you a rounded player, giving you many more options to create interesting musical lines as opposed to simply playing sequential scale/arpeggio runs.
Here are some examples:
G minor pentatonic string skipping
Play this up and down; in different keys; and using different inversions (starting on different intervals) of the pentatonic scale; play it well slowly before trying to speed up.
When your confident try skipping 2 strings sometimes or more; see how it sounds in contrast.
G minor arpeggio string skipping 1 string
Again use different chords, try and find the arpeggio in different positions on the neck.
G major arpeggio string skipping 1 string
Try and make a progression with these arpeggios for instance if you move the G minor arpeggio onto the next string down you get a C minor arpeggio; and move from there up two frets and play the major shape you get a D major arpeggio, try with different progressions in different keys.
Eventually you might start extending the triads for some more colourful options.
EDIT - for clarity here is an example of the progression suggested above:
The chord boxes are just there to illustrate the chords being used, please ignore them when playing the progression (or even better record them as backing and play the arpeggios over them), im still figuring out this jTab thing and haven't figured out how to put a simple label at the start of each arpeggio.
I practice one called in-and-out
Repeat ad naseum.
I have tabbed it using just the open strings, but you can play it against any six string chord. Then you can switch the chords as you play to get a nice little melody going. I usually jump back and forth between Em and Am.
If you visualize your guitar's strings, from lowest to highest, you've got six strings (usually).
Cross-picking or string skipping, means you're going to come up with patterns that jump over an intermediate string.
At its simplest, you could hit the low-E with a down-stroke, then the D-string with an up-stroke, then the A-string with a down, the G with an up, etc., moving across the fretboard. Your left hand doesn't even have to be involved to get started, so fill it with a hamburger or your favorite finger food and feed your face while you train the right one. :-)
If you look at that pattern, and then think of your regular picking hand's motion as you strum or do a single note alternate-picking run, you'll notice a familiar sweeping back and forth motion, which is 90% of what you need to do skips. All that's needed is adding some distance and accuracy.
You can do the opposite of the previous pattern, by starting at the high-E with a down stroke, the G with an up, the B with a down, etc.
From there, work in going across and back, building speed sloooowly, maintaining nice even pace and making sure you maintain alternate down/up strokes. That last part is hard because this is working against muscle memory. When you hit the high-E you should be on a down-stroke if you started with a down-stroke on the low-E, following with an upstroke on the G string as you start the descent.
Then, once you are comfortable with alternating the strings, you can reverse your picking stroke direction on the first note, so your pick is moving opposite the first way.
I show it with open strings, but it can be done with barre-chords, open strings, melodies, or whatever you need.
The goal is to build accuracy and dexterity, starting slowly, then building up speed so you are comfortable and smooth as you move.
Following that, you do two-string skips or other variations.
The goal is to be able to confidently, comfortably, and accurately, jump across strings at will. Once you can land on your string in time with the rhythm of the song you are playing, then it's a simple matter to weave the skips into solos, melodies, simple muted comping of chords or anything else that comes to mind. Coming up with new ways of skipping can become a signature sound for you. Try them with some good digital reverb on a long dwell, or some echo set to a dotted-8th-note, kind of like U2's Edge.
I use an exercise I've heard called "the spider". It basically looks like this:
Then you repeat this pattern with different pairs of strings, gradually spreading the strings apart:
etc. You should also play it in different positions to help your fretting-hand fingers develop a good feel for the fingerboard.
You want to take this exercise slowly and be disciplined with your picking technique. As with all technique development, slow and clean will, over time, pay off way bigger than fast and sloppy.