Well, I swear, whenever I sit down to play, I need a bathroom break within the first 5 minutes. I think my body's figured out I'm going to be sitting there for a awhile so might as well take care of that before I get too settled in.
Also, if you saw my gaming area, you're see my collection of 48oz cups on my little cart I use to lay out my stuff while I'm gaming. I load those cup up with ice, then pour water, tea or Coke over the mound of ice, so I am sure to drink plenty of water while gaming. Where I live it's pretty dry already and I get serious headaches when I get dehydrated
I try to plan ahead, and set specific time to play it. Also I try not to play in the late evening, because then I'll be up all night and my cats don't get fed until 1 AM.
Also, any responsibilities I need to take care I do that first, then play Skyrim.
One thing to keep in mind: it's so much more fun when you can immerse yourself and not worry about needing to go do something else or are trying to play when you're too tired. That's why I plan and try to schedule time that's just for playing it. The nice thing about Skyrim is you're not competing with anyone. It's really all for your enjoyment.
As for gameplay itself, I try not to have too many quests open. That may be hard for newer players because you don't know what to avoid so not to trigger a new quest. But hte fewer the quests I have active, the more easier for me to say "OK, I can stop here and finish this tomorrow" as oppose to "OMG, I need to finish this, and then that, and then that other thing and then go back to X, Y and also Z to get my rewards..."
I also sit down with a relatively specific idea of what I want to do in the hour or so i set aside--maybe I want to finish a quest or two. Or check out that cave/ruins I passed a few days ago. Or travel to the cities and do some favors. Or go hunting or roam the countryside and not worry about any quests. Or just do some alchemy or seek out a spell book. Or try out a different follower. Or work on my Hearthfire house. Basically have a plan.