There are a few I like to do myself. The first one I do is I would hit the notes on the fretboard in a 1-4 finger pattern, starting on the first fret, starting on the sixth string to the fourth string, taking half-steps up the neck untill I'm in the 12th position hitting the 12th and 15th frets.
e|------------------------------------------------------------------
B|------------------------------------------------------------------
G|------------------------------------------------------------------
D|-----------1-4------------2-5------------3-6------------4-7---
A|------1-4------------2-5------------3-6-------------4-7--------
E|-1-4-------------2-5-----------3-6-------------4-7-------------
And so on
Another one I'd do is just start on the first fret of the first string and hit the frets with each of my fingers as fast as I can, 1, 2, 3, 4, then 4, 3, 2, 1, go up down string, 1, 2, 3, 4, then 4, 3, 2, 1, down one string, 1, 2, 3, 4, then 4, 3, 2, 1 untill I get to the first string then go in the opposite direction back up, 4, 3, 2, 1, then 1, 2, 3, 4 untill I reach the sixth string again, take a half-step to the second fret, same thing, half step, third fret, same thing and so on untill I reach the 24th fret...
A third one I like to do I got from reading a website on Joe Satriani's method of learning scales: I'll take a scale and go up and down a string at a time playing only that scale. For instance, the C Lydian scale would be
e|-
B|-x
G|-x
D|-x
A|-x
E|-0-2-3-5-7-8-10-12-14-15-17-19-20-22-24
then backwards back down to the open string. I'd play that a few times then move down on string, play the scale, down one string, etc. etc.
Or you can just play the scales, or an easier song. I usually play John 5's song "Sugarfoot Rag", "Fiddlers" or "Damaged" to warm up, or play Eric Johnson's "Cliffs of Dover". Sometimes I'd just play a scale, hopping around the frets for a few minutes to warm up.
In most cases I can't just pick up a guitar and just start playing my best without any sort of warmup. It usually takes me about 10 minutes to warm up before I can just sit there and play.
Hope this helps....