Every time I play guitar I'm aware of how my playing that day compares with my playing the day before, or the month before. And because life can be so distracting, and I don't always get to play as much as I'd like, the difference from day to day or month to month can be drastic. After giving in to temptation too often, though, and wasting time watching a "Three Stooges" marathon instead of working my fingers to the bone on a stiff set of .011s (that's right, I'm somewhat masochistic), I realize during some solo that my fingers are getting slow. And tired.
Then I know it's time once again to work my scales and arpeggios. Usually, however, I'll warm up with a series of exercises that anyone can do, whether or not they know what a scale or arpeggio even smells like. These can be done while you're watching TV, or staring out the window watching the grass grow. They're easy, mindless, and they'll improve your agility, strength, and speed like nothing else.
Here's a collection of simple exercises for everyone from beginner to pro:
Play F at the 1st fret on the sixth string with your first finger. Then play F-sharp at the 2nd fret with your second finger. Now repeat the pattern on the fifth string, then the fourth string, etc. After you play this pattern all the way up to the first string, bring it back down, second string, third string, etc. Then move up one fret, to play the 2nd fret on the sixth string with your first finger, then the 3rd fret with your second finger. Go back up and down the strings. Keep repeating this pattern, moving up one fret at a time, until you run out of neck. Play this exercise using alternate picking. Alternate picking means one down stroke, followed by one up stroke. Then move to the next string and do one down stroke, followed by one up stroke, etc.
Then start over at the 1st fret using your second and third fingers. Play all the way to the top of the neck, then do the same exercise with your third and fourth fingers (you can start on the 3rd and 4th frets when you do the exercise with your third and fourth fingers). You might even want to do this exercise with your third and fourth fingers twice, since your fourth finger is probably your weakest, and needs the most work.
Repeat Exercise 1, but this time, play F at the 1st fret with your first finger, and G at the 3rd fret with your third finger. Repeat up and down the strings, all the way up the neck. Then do the same two-fret exercise using your second and fourth fingers.
Now repeat the exercise again, this time playing with your first and fourth fingers, three frets apart. Play F at the 1st fret with your first finger, then G-sharp at the 4th fret with your fourth finger.
Play F at the 1st fret on the sixth string with your first finger, then stretch to play G at the 3rd fret with your second finger. Repeat across all the strings, all the way up the neck.
More stretch exercises: Play F at the 1st fret with your first finger, then G-sharp at the 4th fret with your third finger. Repeat as usual.
Even more stretches: Play F at the 1st fret with your first finger, then A-natural at the 5th fret with your fourth finger. Repeat.
After you've worn yourself out with these six exercises, turn 'em around and play 'em backwards: Start on the first string, and play the higher of the two-note sequence first, then the lower note, then move down to the second string and repeat. Continue down to the sixth string, then back to the first string, and then all the way up the neck. If you don't have time to go through all six exercises both forwards and backwards, alternate. Play them forward one day and backwards the next. That oughta keep ya busy for awhile. Oh! The Stooges are on. Gotta go.