I've been leaving my dd, now 5.5, for the whole day since she was a wee babe, first with a friend, and since 2.5 years, at preschool.
One thing I learned the hard way was not to sneak out without saying goodbye when you see that the child is engaged in something else. It is momentarily easier for the parent, but (obviously in retrospect) it makes the child suspicious of getting engaged in anything, and I think it undermines security and certainty. The child thinks she knows the basic lay of the land (mom is still here) and then she turns around & everything is different than she assumed.
Better, instead, to brave the moment of tears or protest, than to avoid it by withdrawing unseen, only to make your kid wonder all the time if you are going to vanish without warning. Not sure why this was not clear to me at the time, but I too was a wimp about the separation tears.
I've also learned over time that dd does much better when she has a lot of advance warning with which to process things that unsettle her. She's gone on some trips with her dad & been away from me for 7-10 days at a time several times. Each time she gets very upset a while ahead, and then sort of steels herself & says goodbye to me in a meaningful way a few hours before the actual departure. When it's time to actually part, she's all cool and blase about it. I actually really admire how she handles such things & it has led me to stop avoiding difficult issues with her. Instead, it works best to get them out & let her deal with & process them in her own time & fashion.
For any crying and despair when you actually leave (this ended more than a year ago at dd's preschool, but thinking back), I found that it is extraordinarily helpful for a teacher to come & physically embrace your child and gently both reassure her that someone there sees that she is upset & cares, and then also to lead her in the direction of an interesting activity as soon as you leave.
For us too, the Kissing Hand was great. I did not read the book with dd but we just made up our own kissing hand ritual which she needed until recently. I kissed her hand, folded her fingers around it, & she kept the kiss all day in case she needed it.
Finally, I concur that knowing what you are doing while she is at school, why this is necessary or a good thing in your opinion, and exactly how & when you will be reunited, is really important.
Good luck & hugs!
Loving Lucy Amelia
"light makes it better"