I would say pick a few of the acts that are must see for you and get yourself there in plenty of time. The ideal situation is if there are a few on the same stage one after the other so you can just stay in one place and enjoy them all. Getting from place to place takes infinitely longer than you ever think, especially if it's muddy. I saw loads of great acts on Friday last year and then didn't really bother with the main stages again much. Unless you're really really really into the music and whoever happens to be on, I'd totally agree that you shouldn't spend too much time at the main stages. There is sooooo much other stuff going on elsewhere. I only saw ONE act the first year I went and still had the best time of my life! Having said that, watching someone you love (like Rolf) at Pyramid with another 200,000 or so people is an experience you don't forget in a hurry. So decide on who you MUST see, go, enjoy, then just go with the flow for the rest of the time.
Personally I love the Healing Fields. It's a great place to chill out and re-align your chi or whatever during the day. At night it's still a pretty cool place and the fire pit is one of my fave places at Glasto.
You should definitely get up to the Stone Circle at least once at night. I missed that the first year and last year all I could say was "this is mental, why did no-one tell me about it earlier?" !!! I howled at the full moon standing on the stones
I also love the Water Aid toilets at the bottom of that field, my fave bogs in Glasto! A nice toilet experience at Glasto is not something to be sniffed at!