Agree with all the pp's but have some other suggestions. Amidst making sure the kids are entertained and happy (clothed and fed too) my biggest mistake has not been thinking about what I need as well. So having scheduled time for you - maybe once a month/week/fortnight whatever you can manage but a set time to do something without the kids. I am also trying to walk before DH leaves for work when possible so I have had some peace and a chance to think/stretch my legs. I have also been to borrow some books for me when we go to the library so I can have an escape also maybe not often but it is there and works for me.
I also try not to continually clean up/do housework etc - have allocated time to do this so the day is not eaten away with it all.
GL xx