One way is to have your spouse take the kids for a day. If your spouse works outside the home while you stay home with the kids, that doesn't mean you deserve a seven day workweek every week. Come to an agreement about when you can have time for yourself. The amount and frequency will of necessity depend on the hours your spouse works. If your spouse or significant other is working two jobs to keep you home with the kids, you can't expect him to give up his one day off too often so you can go relax. Keep it fair to both of you.
If that simply cannot work for you, no matter the reason, try making arrangements with a friend. Trade babysitting in the middle of the day if you're both at home. The kids will get to play with each other, and you'll get a break without spending money on a sitter. Make sure you do the same for your friend. You may also be able to make arrangements with family members.
Another option is to take time after the kids are all in bed and asleep. Even a half hour soaking in a bubble bath can work wonders for frayed nerves. Set up your bath in whatever way you find to be most relaxing - candles, music, books, etc.
Sometimes, of course, the need for time for yourself will hit in the middle of the day when no one is available to help. What you do then will depend on the age of your children.
If you have an infant, there just isn't too much you can do. If they've been crying enough to get to you, you may have to walk out of the room for a few minutes, but make sure the room is a safe one first and do not go far. Take a few deep breaths and relax.
If you find that isn't working, it may be time for a change of scenery with your infant. Go for a walk, sit in the back yard, just get out of the house for a while. You may not even have to drive to find a better place to enjoy your infant. Just remember, they are infants for a short time only, and while it may be dragging now, it will later seem to have flown by. Enjoy it as best you can.
As children become toddlers, it becomes more difficult in some ways to get quiet time when you just need a break. You have to create a much safer area for them to play and ensure they stay there. For some children, their bedroom is simply ideal for this situation. A childproofed bedroom with some special toys can keep a toddler busy for some time.
Older children, can, of course, be sent outside to play when you need a break, weather permitting. It's not only a good breather for you as a parent, it gets them away from the TV and video games and into some exercise. How much you need to supervise them will of course depend on their ages and your comfort level.
It may not take a huge break from your children to make you feel much better and more relaxed. If the stress of parenting is getting to you, take some time for yourself, get relaxed and think about why you are getting stressed. You might just come up with a solution for your family.