A resistor in series with D3 will speed up the turn off time of the relay by dissipating the energy in the coil more quickly. The faster off time will cause the contacts to open more quickly and reduce the arcing.
A snubber network across the contacts or motor will limit the back-EMF generated by the motor's inductance when it's turned off when the current is at the peak part of the AC cycle. The capacitor and resistor will form a damped resonant circuit with the motor's inductance which will reduce the peak voltage and reduce arcing.
by the way i have a question.is it ok to operate the circuit 24x7 ? i mean i continuously supply power to transformer and it will keep NE555 in standby mode so when i press the button it will switch on the relay and after time up it will switch off and go to standby.
is there any danger in doing this or will this reduce life time of the unit?
thanks
Going by the size of the transformer, the size of the box and that you're not drawing too much current from the transformer, you shoudn't have a problem.