A permanent fix for lawnmower tires?

I've owned several riding mowers over the years,and all of them have one problem common to all of them. The tires go flat after a day or two.Every time I want to use it, I first have to air up the tires. I tried that tire "slime" sold at the auto parts store but that stuff doesn't hold up very long, I still have to air up each time. Are there any better tires to be had?