I've found that there can be a few things that might help when dropping my kids off at daycare. First, although I'm sad too, I don't let them know that. I always have a positive attitude about their daycare. I am confident that it's a safe and fun place to be. So, when I'm leaving, I do say goodbye and blow a kiss, but then I just leave matter-of-factly with a smile on my face saying, "See you later! I love you!"
Also, it helps if the sitter can get them engaged in something - whatever their interests are. Sometimes it's a special puzzle, or instruments - coloring - whatever! I do try to say "goodbye" like I said, but if the kids are having trouble that day (it's usually my daugther - my son tends to be more mellow / easy-going) I might slip out quietly rather than disrupting the activity the sitter's got them engaged in. A lot of times, when I do this, my daughter will look up and smile saying, "Bye Mama!"
I find too that sometimes it helps to talk about what we'll do when I pick them up. Will we watch a special show? Draw a certain picture? Play outside?
And, finally, lately my daughter likes me to make her an "I miss you" card. I'm not sure where she heard of this, but we draw it together in the morning, and then I put it in her bag so she can have it for the day. Sometimes I make one for her the night before as a surprise in the morning.
Hope this helps. Good luck!