I thought I'd throw this fun topic out to the forum group, as I'm always interested in learning different approaches that might help add variety to my development as a flamenco guitarists.
As for me, I'm new to flamenco guitar, but have been playing guitar most of my life (59 years old). As you can imagine, I've developed a lot of playing habits over the years that are conducive to good flamenco technique. One real challenge I'm dealing with is learning how to consistently relax, so my warm-up routine is all about focusing on relaxing both hands, barely fretting the left hand positions, and keeping the right hand and arm loose and relaxed
I'm lucky to have an excellent flamenco instructor who I meet with once a week. We're using Flamenco explained for a lot of my practice material. He's a formally trained classical guitarist who also LOVES flamenco and he's able to deliver the goods in both genres. He showed me the following warm-up exercises, with the emphasis on staying relaxed, breathing consistently, and staying focused:
*First exercise is an arpeggio/picado sequence following an Am-G-F-E chord structure. In between each chord shape I play a two picado notes on the b string.
*If memory serves me right, the second warm-up exercise comes from Pepe Romero and it's primarily a picado drill moving up and down each string, starting at 1st fret bottom E and playing left hand i-m-a-c-a-m-i-c-a-c-m-i-a-i, play that sequence at each fret moving up, switch to the next string when you reach the 12th fret and start the sequence moving down. I do this with my metronome set at 160bpm.
So, both exercises take about 5-10 minutes. That's my warm-up!