Relay Protection (Mechanical)
Typical relay failure mode probability:


Failure to Trip 55%
Spurious Trip 26%
Short 19%


As you can see, relays most commonly fail in the "stuck 
open" position where the mechanical switching element fails to close and the relay fails to carry a current. Relays are less likely to unintentionally close or remain closed after the 
switching current is released. However, high voltages and current can actually spot weld the relay in the closed position.

A major cause of early life failures in relays is 
mechanical wear of internal switching elements. In fact, the life of a 
relay is essentially determined by the life of its contacts. Degradation of contacts is caused from high in-rush currents, high sustained currents, and from high voltage spikes. The source of 
high currents and voltages, in turn, are determined by the type of 
load. Inductive loads create the highest voltage and current spikes 
because they have lowest starting resistance compared to 
operating resistance. This is especially true for lamp filaments and 
motors, which is why derating is more severe for these types of 
loads. The life of a contact can be further degraded if 
contamination or pitting is present on the contact. Physical wear 
can also occur to other elements within the relay. Some relays 
contain springs to provide a mechanical resistance against 
electrical contact when a switching current is not applied. Springs 
will loose resiliency with time. Relays can also fail due to poor 
contact alignment and open coils. 

So in order to protect the relay, just avoid contamination of the contacts, high in-rush currents, high sustained currents, voltage spikes, and inductive loads.

Found this nice PDF (brought to you by the military): http://www.navsea.navy.mil/nswc/crane/sd18/Public%20Documents/ProductArchive/Relays.pdf