I looked some up but there was nothing in much detail, just things like "bend your wrist back" and they didn't work. I do a scale, GFEDCBAGFEDCBAGFE that just uses around the first 3 frets on all 6 strings, going up and down once or twice, and it loosens it up a bit, but even before I pick up guitar, have you got any exercises that you can do to prevent pain? in detail if you can, thanks
Warm up first, shake out your arms and shoulders till they tingle down to your fingertips. Make sure you play with correct posture, that'll help vastly - vid in my sig.

And always be on the lookout for tension of any sort.
For starters, make sure you warm up well. I shake my arms and shoulders, then do some stretches on the guitar from John Petrucci's Rock Discipline. You don't need the book though, all you do is just play something that causes your left hand to stretch (not so much that its painful though). Then, I do some string skipping with my right hand, and some chromatic alt picking to sync my left and right hands.

Make sure you have good posture when you play, and in regards to tension-if something hurts in ANY way-DONT DO IT!
i usually start by shaking my wrists around to loosen them. and then get comfortable with myself do some scales and im good to go.