What do I do to warm up?
Develop your daily rituals and "Kata's".
I talk a lot about practicing everyday; and I cannot emphasize this daily ritual enough. Practicing your guitar, (not playing), should be thought of as a “workout”. Just like physical-workout exercises you do for your body, it is always best to warm up before you start.
Tune your guitar first!
The first thing I do is check to see if my guitar is actually in tune.
Once I'm tuned, I will almost always go through a few scales. I normally play these scales (or modes), right in the middle of the fretboard. I don't necessarily feel a need to over-stretch my fingers on the lower frets. I like to play the phrygian scale from top to bottom. I will do variations on the way I play the notes in the scale.
I don't normally use the metronome to warm up - because I'm not really following any sort of rhythm like I would if I were learning a song. This is just a warm up to get my fingers and hands operational.
Modes/Scales
I will sometimes go through all seven modes: (Ionian, Dorian, Phrygian, Lydian, Mixolydian, Aeolian and Locrian), starting each mode with the same root note.
The easiest way to train your ears for this is to play the corresponding chord one time for each mode.
For example, if I am playing every mode starting with "A" as the root note, I will play the following chords that correspond with each mode:
Chord/Mode/Notes
A = A Ionian = A, B, C#, D, E, F#, G#
G = A Dorian = A, B, C, D, E, F#, G
F = A Phrygian = A, Bb, C, D, E, F, G
E = A Lydian = A, B, C#, D, E, F#, G#
D = A Mixolydian = A, B, C#, D, E, F#, G
C = A Aeolian = A, B, C, D, E, F, G
Bb = A Locrain = A, Bb, C, D, Eb, F, G
I will do each of these modes one time.
You can find the tabs for all of these modes here.
Cowboy chords
Next, I'll play through a handful of open string "cowboy" chords: C, G, D, A, E, F, Am, Em, Dm, Bm, etc. I will then go through a handful of jazz chords that are not "barre" chords. I'll play the 6th string root Major 7th, then maybe an 11 and a 13 chord, just to stretch out my fingers for these shapes. I might play a 5th string root 6/9 chord or a handful of other inside or cluster chords. I call them "inside" chords, because they use the four strings in the middle of the fretboard, not the 1st or the 6th strings.
All in all, my warm up might last a few minutes. I really don't need to be aggressive about it or spend a lot of time doing the warm ups. When you play and practice your guitar every single day like I do, there is a level of ability you try to maintain. For me, this level is being able to play three, four or more hours each day without being in any real pain or feeling any discomfort in my fingers or hands. I'm happy with being at about 75-85% ready to play any gig at any moment. You always want to be ready for any possible scenario.
Kata’s
Warming up is important - because you need to stretch and go through your daily ritual of what the Japanese martial arts refer to as "Kata’s".
In martial arts, "kata" (型 or 形, meaning "form" in Japanese) refers to a set sequence of movements, practiced solo or with a partner, to learn and perfect techniques and build muscle memory.
Practicing your guitar every day means training the muscles in your fingers and your hands, to perform as fluid and precise as possible. There simply is no shortcut for this. There is no such thing as an "easier way". Yes, your fingers will get tired and sore. So what? They're not going to fall off. Play through the pain. Of course, you don't want to overdo it and injure yourself. Be smart. Warm up everyday with some kind of ritual before you practice and before you play your guitar.