Using seperate power supplies is certainly best way to deal with this problem.
BUT
if you are facing same problem as i do i.e. PCB is made but problem arise after or you do not want to waste any more of you time for problem which can easily be solved with a little trick. Then I think you take my advice and connect a diode and capacitor, to isolate the power supply and RESET pin of mcu from the main supply.
It is is just a quick trick that i usually used and due to this system will amazingly work on higher noises and disturbance levels.