I think it is crucially important for SAHMs to get weekly breaks at the very minimum! You should get some time when you aren't responsible for the life of another person besides you. You should know when this time is so you can know it is coming when you are stressed (only one more day til Wednesday!!!)
Things that I do that are SO wonderful for my sanity/happiness:
grocery shop alone, often at night after kids are in bed (and dh is home of course). I can read labels, check prices, even read through the cards to find the ones I need!
work out at the gym. Have just started this a month ago and it is amazing the difference in me! Physically and attitude-wise! I meet a small group of women friends there at 9 p.m. four nights a week. It is just more fun than you should be allowed to have while exercising!
listen to music that isn't Barney or Raffi. Now I like Raffi just as much as the next person (actually I like Red Grammer more but that is beside the point), but listening to Gavin DeGraw or U2 or Alicia Keys.... I can't really describe what it does for me emotionally.
spend some money and time on things that are JUST FOR ME, absolutely NO connection to my kids/parenting at all. So time at the PTA meeting, or scrapbooking pix of the kids doesn't count. Yes, that is time away - but still child focused.
What is for YOU??? Buy yourself a cute new pair of shoes. Take that art class you read about. Meet your sister for lunch. Call a friend who always makes you laugh. Go to the library (alone!) and look through the biographies or mysteries or something OTHER than parenting books and Dr. Suess. Join a book club or bunko group or recipe club - anything that gives you an excuse for a once a month outing just for you.
I am in the local MOMS Club and that has been great for me and my kids. It includes a once a month moms night out which varies from month to month. Sometimes a movie, sometimes game night, sometimes a dinner out at a restaurant, once it was rock climbing! MOMS (Moms Offering Moms Support) is a great SAHM club with yearly dues of only $25 - and it offers SOOO much in the way of playgroups, holiday parties, chances to volunteer civicly (that isn't spelled right, is it?) with your kids, plus the moms night out - it really is the greatest thing I've done to make friends and feel supported as a stay at home mom in my town.
It took me too long to figure out that I REQUIRE time when I am not on call, when I am not the one wiping noses or filling sippy cups. I am a happier person, and therefore a much better mom, when I am not emotionally drained, depleted, stressed.
Take some time (how much will vary with the age of your child, if you are nursing, etc) for yourself and don't feel a lick of guilt! If you can get your dh to understand you need it and he is home with your dc, great! If not, call in a gramma, auntie, trusted friend/neighbor to get your needed time. If you don't take the time you need, it will end up badly eventually. I completely agree with the "if mama ain't happy, ain't nobody happy"! Go find some fun stuff for you!!!