Sounds like the main relay is starting to go. They tend to die in the hot summer heat. The main relay turns on the fuel pump, and when the relay dies... no fuel. When you turn the key to ON, you should hear the fuel pump build up pressure in the fuel system. Two seconds or so? If you don't hear that, your main relay isn't working. They go intermittant for a while, then finally die altogether.

If it won't start ONLY when the ENGINE is hot (ie., when it's up to normal operating temp), it's probably the coolant temp sensor.