From what I've heard/read etc the key is not drinking too much during or even before you run but rehydrating afterwards.

The key line to take is if you're thirsty- drink! If you're aware your mouth or throat is dry, take a sip, but remember you're not refueling so much as topping up- have enough to moisten your mouth and throat, but not enough to make yourself slosh! After your run, have a few mouthfuls and then wait for a moment before repeating- you want to make sure you're rehydrated without making yourself sick from having too much water too quickly (this is especially important if your stomach is the kind that's inclined to feel queasy when you've pushed yourself a bit!- Mine is, and I'm definately guilty of trying to drink an entire 2-litre bottle of water to quench my thirst after running only to instantly regret it!)

I think the secret is that your first couple of mouthfuls of water after running shouldn't simply be gulped, but held in your mouth for a second- maybe even go so far as to gargle, so that when you do come to rehdrate you're not influenced by how dry your throat feels. After that, it's a case of gauging how much water feels like "enough"- generally speaking our bodies are pretty good at telling us what we need- drink until you feel sated, but at the same time don't allow yourself to drown!