Does the starting problem only happen when the engine is warm? As in, you have recently turned the motor off and want to turn it back on again, but it won't? If that's the case it's most likely a gunked up fuel filter. You also could be having issues with the injectors and possibly the pump.

The first thing I would do is check to see if the fuel pump is turning on when the car won't start. Have someone turn the key to the ON position but don't start the car and stick your ear up to the fuel filler (yeah I know, you'll feel like an idiot doing it ) to listen for a noise. The pump should run for two seconds when the key is turned.

If the pump is switching on I would then replace the fuel filter and run a bottle of fuel injector cleaner through the next tank of gas.