Tires are affected by heat, UV light, and age. The heat and the UV light age them faster.
The rubber loses it's volitiles. Those are the petroleum products that can evaporate over time. You can recondition that but those compounds are expensive to add so it's better to protect them.
Cover or shade the tires so they don't get direct sunlight. Use 303 or some other protectant on the rubber to help them handle the light. Just make sure it gives UV protection.
Tow the trailer around instead of letting it just sit there. The tire will last longer if it is used than if it is stationary for long periods.
Check your trailer brakes and look at the trialer tires for cupping or feathering which might indicate alignment issues on the trailer. And check or replace the valve stems as lakeski suggested.