M.D.
I start looking now for summer. I also use this person during spring break and on date nights to get the kids used to them. When we were looking years ago for our morning sitter, we started looking in late July/early August. Fortunately, we interviewed quite a few people. The one we chose for full-time care was HORRIBLE and she was fired almost immediately. The second one we hired just to do the mornings and she is still with us. She even did last summer for us as well. I need to talk to her, but I do not think she is going to do this summer or next school year, she is 22 and ready to move on.
Anyways, start looking now (we used sittercity.com) and start using that person here and there so everyone gets comfortable.