Start by pulling the toilet. Clean out the 3 or 4" line going down from your toilet. Check the toilet itself for any obstructions in the trapway of the toilet. (You might need to run a toilet auger down through it.)
Over time, scale and other mineral deposits can build up on the inside of the trapway of your toilet. If there are no obstructions in the toilet or the main line and you still have problems, consider replacing the toilet. These days, we recommend the American Standard Cadet III toilet as it is a great flushing toilet for the money.
http://americanstandard-us.com/innovations/flushright/