Random microcontroller deaths?
There are several reasons why microcontrollers would act as described above. The three most common causes are:


You are trying to supply too much power. Putting 12 volts into a 5
volt chip will kill it instantly, for example.
Your loads are drawing too much current. The microcontrollers can
only source a certain amount of current, and trying to draw more
than that (for example, trying to run a motor directly from the
chip's output) will likely cause it to burn out.
Shorted pins


To fix these issues, try the following:


Most microcontrollers require either 3.3V or 5V to operate. Check
your chip's datasheet to make sure you're not supplying too much
power.
If you're driving large loads, like motors or coils, use a transistor
connected (through a base resistor) to the microcontroller. Then you
can control the transistor with the microcontroller, but the load
will draw its power from the main power supply instead of from the
chip.
Check that your wiring is correct. Ensure no pins are touching, that you don't have any solder bridges (if your chip is soldered into a circuit), etc. Make sure the schematic is followed exactly.


The problem you are facing most likely stems from one of the above problems, so check through them and see if they help.