You might try listing your rental on Snowbirdseeker.com because they'll provide you with renter matches, so rather than just posting you can actively search for a renter. Other options would be to list on craigslist or homeaway.com.
Ackley Florida Property Management, http://www.AckleyFlorida.com is a Kissimmee Prop Mgmt company and they utilize rentclicks.com/rentals/com to showcase your property as well as many other online & print locations.
I've found in the Kissimmee area that a site formerly called RentClicks.com -- now Rentals.com -- is the most popular site among those who rent. I think it's because of heavy public transportation advertising. It's not free, like CraigsList, but is safer and offers good results. If you need a property management agency -- not necessarily an agent -- consider Overton Realty. They're established with experience throughout Kissimmee. Go to OvertonRealty.com for more information.