My feeling about plaid in fall is the more the merrier. It comes in great colors, cozy fabrics and classic, flattering cuts. However, plaid can also teeter into lumberjack (and grunge band) territory. There are a couple tricks to avoiding this trap.
You can always pair plaid with something girly like a skirt and heels (a la Monday's post). But the best way to avoid a bad plaid situation starts at the store. A lot of plaid shirts have breast pockets, and some have buttoned breast pockets. One regular pocket is ok, but I shy away from versions with button pockets on both breasts. It instantly makes the shirt look more utilitarian (or western) and generally less flattering.
Here are some of my favorite plaid shirts for women.
Depending on your size, you could also go for a Zara boy's shirt. I think their size 13-14 is pretty similar to a J.Crew XS or S, and they are a great deal at under $30 each.