I also believe that a little outdoor time and exercise does wonders... even if it's just a walk around the block in the evening.
For me, a little "alone" time helps, too. Sometimes I just ask my hubby to take the kids out for a few hours. While he's gone, I occasionally take the time to relax or read... but mostly use the time to get stuff done: a quick house cleaning, catch up on bills or email, etc. When they come back, I'm refreshed, and feel like I have a better handle on the "to do list" that's always in the back of my mind.
I also really make an effort to focus on the positives... I look at the joy I get from my kids and imagine what I would feel like if I didn't have them. I look at the work I have to do each day, then imagine the pressures of moms in underdeveloped countries, and the fact that no matter what troubles I think I have, it's nothing compared to what they go through simply trying to keep their children safe and fed.
Finally, there's an old saying "Fake it til you make it." Basically, the premise is, if you "act" happy, you'll become happy. So when I get into a funk sometimes, I'll put on a happy face and before I know it, I'm having a good time.
Good luck!