During the 10 years of my youth that I played guitar, I pretty much warmed up by playing Master of Puppets or something.
Now that I am actually getting better than I was back then, and now that I am older (31), I find that I am pretty useless if I don't thoroughly warm up.
My warm up is 5-10 minutes of random finger combination + alternate picking and sweeping.
Then I go through the only modes I know: ionian, aeolian, and lydian. I'm so proud of myself cuz I finally understand that C Ionian = A aeolian = F lydian. I need to learn the rest now.
After enough warmup, I begin to play real songs by playing the intro to Twilight Symphony by Stratovarius (basically a descending harmonic minor plus a few chords).
Once I hit my groove with that, I am warmed up.
I know I am there because my fingers start to feel lighter than before. In fact, this is new to me. I remember a few weeks ago I was playing and my fingers just suddenly starting using a much lighter touch and I started playing faster. I was like wtf just happened, this is awesome!
I must go through this ritual, even in guitar stores. So I sound like the biggest noob for the first 5 or 10 minutes when I'm testing out guitars. And that's a big deal because you know entering a guitar store puts your ego into overdrive because all the people there actually care how good of a guitar player you are
Seriously tho, I need to warm up, and that's never been the case for me before. Anyone else?
Normally if I'm just doodling, I won't warm up. But if I'm seriously practicing or performing, I always do an extensive warm up of both my picking hand, fretting hand, and their synchronization.
When I was in college and played mainly trumpet (didn't have time for guitar as much), warming up was a huge part of my practice routine. One of my trumpet books said that one's warm up should be at least 1/4 as long as the duration of the practice. For example, an hour long practice should consist of at least a 15 minute warm up.