What country are you from?
If you can get 30 days free or not depends on the country you are from and the passport you are using.
If you are from a country that qualifies for the free Visa Exemption stamp you can get the free 30 days when you show up each time.
Recent change - November 2013. This updates previous post from October 2011.
Tourists entering Thailand overland from G-7 countries now can get a free 30 day Visa Exemption. Previously the free Visa Exemption allowance was 15 days.
This is USA, UK, Canada, Germany, Italy, France and Japan.
[ Edit: Edited on 03-Dec-2013, at 16:19 by karazyal ]