I put my son into creche when he turned 5 months old as I had to return to work.
What I did was on the Thursday before he started I took him, and I sat with him at the creche for an hour, or maybe slightly more. I didn't pass him off to any of the teachers, I just sat on the ground with him where he could hear the new sounds, see the new people, and just generally assess what was going on around him, whilst still in the safety of being close to me (mostly in my lap).
Once I felt that he was relatively content I left him there for 1 hour and I went out shopping. When I got back to fetch him he was fine.
Then the next day (Friday), I again took him. I stayed with him for about 15 min and then I left him for about 2 hours while I went to watch a movie. That was SO hard for me. I kept wanting to rush back and get him but I knew it was for the best, and in the long run I was glad I had the movie "keeping" me there for the full time.
When I went to fetch him he was again totally fine and not upset at all.
Then the monday I had to go back to work so I took him in, stood in the doorway with him in my arms for about 5 min while I showed him where he was and all the people there, and then I passed him to one of the teachers and left.
Each day I found that if I held him for about 5 min when I arrived and then I passed him to a teacher he was mostly fine. On the days when one of the teachers came straight up to me as soon as we arrived and tried to take him from me then he got upset. It was like he needed that 5 min to sass out where he was and that it was ok.
There definitely were a few days of tears here and there over the next few months, but for the most part they were few and far between, and almost always he stopped crying within about 30 seconds of me closing the door behind me.
When he went back at the beginning of this year he had to start in a new class as he was now walking (14 months old). He had been with me for about a month during the xmas holidays.
So again I took him the day before I had to start work and I held him while I chatted to his new teacher and told her his ways etc etc. I let him look around and see the kids and the new play area and where he was etc. Then gradually his curiosity got the better of him and he wanted to go down, but wasn't quite confident enough yet, so I put him down and held his hand while I carried on talking to the teacher. And gradually he let go and wondered off to play in the sand pit.
When I took him the following day and just had to do the quick 'drop and go' so I could go to work he was fine.
One thing I am very particular about though is that I ALWAYS say good bye to him. I never just quickly sneak off while he's not looking. I feel this is important because then he knows that if he turns his head I will still be there when he turns back, unless I have said good-bye and then he knows I'm leaving and that I will be back later.
Also, I only say good-bye when I really am ready to leave. If I say good-bye I leave and that's that. No turning back, no rushing back for last cuddles and kisses. I think that confuses them and makes it harder. If i need to fill in books/forms / chat to the teacher I do those things BEFORE saying good-bye. Good-bye only gets said literally seconds before I walk through that door, and then I'm gone.
There are deifnitely times when I have to grit my teeth and force myself to keep moving, especially on the odd occasion that he looks miserable, but it is necessary.