Using two voltage regulators (780x) in series to prevent heat sink
A better solution is to put a resistor in series with the 7805.  The power dissipation will be shared between the regulator and the resistor.  This will reduce the peak power dissipation in the regulator.

The 7805 has a minimum input voltage of 7V.  If you need 200mA from a 12V source then you can use a resistor of...

(12V - 5V) / 200mA = 25 ohms.

The peak power dissipation in the regulator will occur when the voltage across the resistor is mid-way between 12V and 5V (which is 8.5V).  The current in the resistor will be (12V - 8.5V) / 25 ohms = 140mA at that point.  

The power dissipation in the regulator will be 140mA * (8.5V - 5V) = 0.49W.

Thw peak power dissipation in the resistor occurs at max load (200mA).  

The peak resistor power is 0.2A * 0.2A * 25 ohms = 1W.