Paint walls an adobe or sandstone color, with maybe one wall in each room being a brighter color version of them. Mexican serape material, either in blankets or rugs can be used as throws on chairs or scatter rugs.
There are many alternatives to have a Mexican style room. You can get wall hangings, there are blankets and things from Mexico that are actually cheap and you can use for covers, you can use a tan color on the walls.
Google image search Mexico, mexican style, art etc to get some ideas. Colourful repetitive patterns! Blue and white tiles! Birds! You can get really nice mexican tiles or you can paint thinsg that nice rusty red.