Les Miz takes place mainly in the years 1817, 1823, and 1832. I have seen, however, that many fanfiction authors don't seem to know this! Almost every time I click on ff.net, I see a fic with some historical inaccuracy. I have recently written a historically inaccurate fic. It was the result of POOR RESEARCH. Make /sure/ you research before putting Cosette in that miniskirt!
It's very, very easy to slip into modern day speech when writing dialouge, but remember, it's 1832. They did not say "okay," nor did they say "what's up." Here's a few common modern day words/phrases and suggestions for more accurate replacements:
2004: "Hi", "Hey", "'Sup", "Yo,"
1832: "Hallo", "Good day", "Good afternoon", "good evening".
2004 "What's up?", "How's it going", "You OK?" ----
2004: "Omigod!" "Yikes!" "Bullcrap!" ----
2004: "Okay" ----
Another thing that's all too common in fics is the addition of common 21st century things such as electricity. Remember that electricity was not common until the early 20th century, as well as running water. Again, make SURE to research! If you aren't sure whether or not something was around back then, either look it up or leave it out.
Modern clothing seems to be the number three most common inaccuaracy. I see absolutely gorgeous knee-length dresses all the time, and I know that they are perfectly acceptable today. However, you cannot put your chracters in modern fashion! Women dressed very modestly (though showing much of the breast was allowed), and there is a point in Les Miserables where Marius accidently sees Cosette's ankle and is appalled, mostly because he does not want other people seeing her ankle. Put Enjolras in a t-shirt and I will rip my hair out.
Those are just a few tips for a historically accurate story. There's a lot more information out there, and I've linked to some valuable sources in the links section.