It sounds daft but for a long time when I practised I actually used to use a sort of mantra: back when I was at university I used to do martial arts in my spare time as well, which you'd think was completely unrelated but the relaxation element was almost the same principal: what looks like it takes a lot of effort to do actually doesn't, and this one session I remember doing the instructor talked about 'Minimal effort with maximum result.' I used to remind myself of that line when I practised guitar as well to remind myself to take a check on how much tension I had going on, not just in the fingers but in the arms as well. The same instructor also said 'when I'm training these moves I keep thinking "Am I relaxed?" ' Another quote I kept calling to mind for guitar practice, it worked just as well for that.
A word of caution on picking though, I found at one point that I was relaxing my pick hand so much that the technique became sloppy, and I found once I put a little more tension back in there I started to get the effect I was after and be able to do the practice exercises I was working on more consistently. As long as you're not gripping your pick or fretting your strings like you've got them in a vice and your hands aren't starting to hurt it's always worth trying a little MORE tension until you feel like you're comfortable (and don't forget the action on your guitar can also make a difference to how much pressure you need from your fret hand (it takes the hand more effort to play with higher action.)
One thing not mentioned here yet is warming up before you play. This can make a difference and reduce the risk of you ending up with sore hands the day after playing. I remember about eight years ago having a go with John Petrucci's Rock Discipline video and before he did anything he took me through warm up exercises and I thought 'Warming up for playing guitar? WTF?' but as this was someone experienced who I admired I tried taking it seriously and realised he was actually right, and not only that I realised it's not just your hands you need to take care of when playing but your arms shoulders and back as well. I admit I often don't warm up before I play but if I'm having a long session I tend to take a ten minute break and do some exercises to ease muscle tension, and since I started doing that I get far less aches the following day after spending 4 to 5 hours playing.
'There's no such thing as bad weather, there's only the wrong clothes...'
