When I suggested a cowboy theme for my 5 year old's birthday party, he chose to call it a "Wild, Wild West" party. I found alot of western clip-art and let him choose some graphics
for the invitations, then designed them on a desk-top publishing software. We hit several "dollar stores" in the area and found colored straw cowboy hats; toy sets with a gun,
sheriff badge, handcuffs and a Lone Ranger mask; and a bandana for each child invited to the party. We decorated plain denim-blue gift bags by drawing a pocket on them with black marker, then
gluing a scrap of red Western print at the top to look like a handkerchief sticking out of the pocket, and we put each child's name on a bag.
We planned several party games using ideas from this great site, plus played "Pin the Mustache on the Cowboy" using a clip-art picture of a cowboy and a separate picture of a mustache, which we printed on a color printer and laminated. The paper cowboy vests made from brown paper bags were a big hit, as were the rope lassos that my husband made for each child from inexpensive rope. We set the table with a red and white checked tablecloth, red partyware, and made cloth napkins by pinking the edges of squares of the same red western print. The kids had a blast!