Easier said than done really but I think it's really mind over matter at times... (unless everyday you feel like killing yourself or someone or everyone at work!). I tend to make it a point to LOVE going to work every morning. I love it that in the mornings I'm the first one there for the normal day shift. There are those from the night shift that would linger on and I'll chat with one or 2 of them. It's also my quiet time to catch up with emails while having my morning drink. Well I start at 7am and people only start trickling in at 9am! so it's a good 2 solid hours of power hour just replying or doing some work quietly.
I spend my mornings also catching up with colleagues even if it's just a short chat. The morning drags on until 11am and I'll be anticipating lunch! Nowadays it's just bread or a snack at the panty, but you know, we need that short break. Talk cock sing song or just to have a meaningless chatter at lunch is really rejuvenating. A break from really serious stuff.
Because not long after that, it's power hour in the afternoon. My afternoons tend to be short since I leave at 4 or 4.30pm. I make it a point to go home ontime because really there's no point in dragging work till late evening when you're out of focus and feeling HUNGRY. Anything can wait till the next day really. One more email is not going to make any difference!
And that's how I stay positive at work. Friends, endless chatter (this has worked in all my jobs!), lunch, and in general just make every hour worthwhile so you dont have to stay till late evening.