I agree; a tied in front plaid shirt is very western. However, depending if you want to go all out rodeo girl, saloon girl, or elegantly refined 1800’s western female, consider all of the options…
For the top, consider a low scooped Wild West halter vest http://s9.thisnext.com/media/largest_dimension/AF364AE6.jpg, a western corset http://www.etsy.com/listing/6195813/western-corset orhttp://img.alibaba.com/wsphoto/v0/474197460/NEW-Arrival-Free-shipping-NEW-Fashion-Sexy-Corset-Top-Corset-Dress-and-G-string-including-Bustier.jpg , a frilly western saloon bodice http://profbloodgood.net/costumes/images/1WTDCDeadwoodCamCorset.jpg, or http://jonahhexadultcostume.com/wp-content/uploads/2010/07/Jonah-Hex-Lilah-Red-Adult-Costume1.jpg, or a Steam-punk undervest http://lillysworkshop.blogspot.com/2010/05/two-new-corset-variations.html over a vintage, off the shoulder, western scull-maid (barmaid) blouse http://www.outwestmktg.com/detail.cfm/A-Scully-Ladies-Vintage-Western-Shirt-The-Steampunk-Peasant-Ruffle-Black/3538 this particular one comes in turquoise, black or white, or there are lots of other western shirts from which to choose.
For bottoms, consider denim cut-offs, a western cowgirl skirt http://www.oldtradingpost.com/old-west-store/bmz_cache/0/0cec60247cb02c11b6e68b3100d87ebe.image.150x307.jpg, or a miniskirt with fringes http://img0.etsystatic.com/il_fullxfull.281369256.jpg. If you do a skirt, don't forget the fringes... they are awesome fun! If you do shorts (cutoffs) consider putting on chaps over them. For accessorizing contemplate a bandana belt, a Sherriff’s badge, turquoise or rhinestone jewelry, guns and holsters, or even a hip lasso (rope tied at the hip; think Wonder Woman http://themaskedblogger.com/wp-content/uploads/2011/03/wonder_womanTV450.jpg). Whatever you decide, have fun with it, but don’t weigh yourself down too much with accessories!

use some overalls and a sports bra underneath with a straw hat and cowgirl boots :)

Being that I was raised in the country, simple an sexy is ur best bet.I agree with Kasey, A sexy tight plaid shirt is a must. Buy it a size smaller so it will show off ur belly .button up just under ur breast, wear daisy duke shorts with ur boots. I belt with a large buckle will look hot. Maybe braid ur bangs an pin them about half away across. This is will allow ur hair to flow sexy yet give it a bit of country look also. Hope this helps. XoXo