I read a book in the early to mid 1990s with this plot:

A girl, either a pre-teen or young teen, plays some game in front of a mirror. Suddenly she can see through the mirror to another girl who looks just like her, except inside the mirror it's obviously the 18th or 19th century. Somehow the mirror girl manages to trick our protagonist to switch places, and our protagonist is stuck in this horrible life where's she basically some sort of indentured servant, fetching things from wells and doing tons and tons of laundry and all the people she lives with hate her. And the mirror girl is now in the 20th century wreaking havoc on our protagonist's life and generally have a great time. Sporadically they can see each other through the mirror, and mirror girl loves gloating. The protagonist has to come up with a way to trick mirror girl into switching back.

If anyone can remember the title of this book, I would really appreciate it! Like I said, this book was read in the early to mid 1990s, so the book is 15 or so years old.

I've Googled around but can't come up with a title for you. Please call or visit your public library and ask for reader's advisory. They have print and electronic resources to help pin this down, plus you have lots of detail to help shorten the process.

Meanwhile, you might like to read The Mirror by Marlys Millhiser --

"On the eve of her wedding in 1978, Shay Garrett peers into the antique mirror in her family's home, the famous Victorian Gingerbread in Boulder, CO. She falls unconscious only to wake in the body of her own grandmother Brandy on the eve of her wedding in 1900. The virginal Brandy, in turn, awakes in Shay's body in 1978, to discover she's pregnant. What follows is a fascinating book how 2 women -- and their families -- cope with this strange, sometimes humorous situation."

Sounds like "charlotte sometimes" written by Penelope Farmer in 1969.

Could also be the film Mirrormask you're thinking of.

Oh My Gosh!! I read this book too, the cover showed her in the mirror, looking out, she had dark hair - I am not kidding you that yesterday I was thinking, 'what was the name of that book where the girl was tricked into getting in her mirror.. ' How random that you were thinking about the same book! It's a mission now - the search is on!