As a mum and a nurse I hear all sorts of stories and I figue there are a lot of people who just are not kid people out there looking after children.  My Mum inlaw is one - thinks she is the ultimate child carer but my son can't stand her sadly, as she adors him and is ever so willing to have him and play with him but her strikt regimented day were they have to pack so much fun into one day that he does not get to take his time with things does not fit with my layed back style of  parenting.  So i guess the best advice anyone will give you apart for the obvious, stuff about good reputation, character etc. is look for some one who has a similar style to yourself and no matter how wonderfull the sitter seams to you if the child is not happoy with them and unwilling to be around them after a reasonable  amount of time for them to get to know each other and the child to get into the swing of being left (ie regular sttting once a week for 3-4 week or irregular then a little longer., then find someone ealse.  Age is not an indicator nor is if they have been a parent themselves ( i'v seen some very average parents), My Foster daughter (was 19 when she baby sat my young sone for the first time on her own and she had never had kids and had the worst up brinning before we got her at 14yrs old and she was wonderful with him.  She has a natural ability with children and most children seam to like playing and being around her despite her rather stricked  rules etc. So go with the flow the gut and what your child is showing you, are they happy  are they ok going to this person.  My Mother inlaw has know my son since birth but  that changes nothing,  She adores him and always brings lots of games and things to do but My son still hates being left with Grandma, he would rather go to child care than stay with her, and she is a wonderful person and very caring and would never hurt my son, but hey he is not happy and that makes for a stressed mum so I make other arrangements ( taking care not hurt Gannys feelings).  Children are funny they need to like the person they are with and someone with your own style of care and system is probably the best bet.
good luck

Hi, I was a Nanny and I know there are a lot of lazy dodgey people out there who are not the sort you should trust with your most irreplaceable priceless child.  Ask around your Mum's group.  Having others taking the first steps for you in my book is better for you.  Someone who has earned a good reputation has to be good at their job.  I always charged less for night time babysitting than day time unless I was puting the kids to bed.  I know that is not the norm if you are going through an agency, but as I was my own boss it only seemed fair to me.  You do less, so you should earn less.  I did wash dishes and tidy up even swept floors, but apparently I was unusual in these practices as well, which is probably why I had a waiting list of hopeful clients and only ever placed 2 adds in 4 years in this town alone.  Word of mouth.  Here there is a babysitting club where parents swap around for each other and no money is exchanged.  If you don't mind re-paying the favour this may also work well for you.  Firstly it's another parent and secondly they can expect simular service from you, so the stakes are high to impress so you may do the same.  Unless it is a friend doing you a BIG favour, they should always come to you as that is where your child will be the most comfortable and try to have the child (for night time babysitting) already asleep prior to the sitter coming, it will be easier, but let them know they will be here incase they wake up and freak out.  Meeting a sitter first in alert daylight time is always a great idea, but not always possible.  Use your instincts.  Good Luck