1. Dont draw power for the relay coil from the same trace as that going to the MCU. If you are forced to do so then adequately decouple the MCU power take of point ( a 10 Ohm resistor is series and a 100Mfd+0.1Mfd caps to ground from the Vcc pin of MCU are examples - but will depend on the current drawn by your MCU etc )

2. The Freewheel diode across the relay coil is a must and it MUST be right across the coil - not 1/2 KM away from it.

3. Snubber across the relay contacts - a series combination of 0.1Mfd (600V rated) + 100 Ohms would normally do.

4. Finally avoid ground loops. In other words all power guzzlers like relays should be connected directly across the power supply point.

The best however is to completely float the MCU 5 V power supply with respect to the relay coil power and use a driver + isolation at the MCU pin for driving the relay. ( This way I have 8051's driving a host of relays in a Plasma Transferred Arc welding unit - no problems whatsoever)

Hope to have been of some help

Raghu