I found my first sitter at Target, she worked there as a cashier and was a senior in high school. My sil works at another Target and I know they do random drug-testing, so that was a bonus! I asked her for some references, I spoke to her boss, and she had her mother call me, which I found very cute. The first time I left my kids with her, my boys were 2.5 ish and I took dd with me, and I was only gone for an hour. All went well and I hired her regularly, like once a week so I could grocery shop or something on my own. I've also had her dog-sit while our whole family went out of town for a weekend. She's a good girl, I like her, she's been our regular sitter for over 2 years now. She cleans too.
I have a few back-up sitters I've used, I usually get recommendations from friends. One girl is a preschool dance teacher and she's very sweet and the kids like her, and another girl works in the Child Watch at the YMCA where I work on the weekends.
Trust your gut, get references, get recommendations from people you know, and start slowly. I think it is nice to know that my kids are with someone new, getting a new face in the house for an hour or two, and they have fun.
Erikamom of (8) (5) (3) and born at home on Christmas day!