While the electrical part of the ignition switch is a prime suspect and
is often the cause of intermittent starting woes, I'd like to point out
other possibilities.

1. If an automatic, maybe the neutral safety switch, in the shifter
housing, isn't making properly.

2. Dirty (but maybe tight) connections.

Remove and clean (clean means make shiny bright) the male/female spade
connection on the starter solenoid where the wire from the ignition
switch goes.

Remove and clean battery cable connections at the battery.

Remove and clean the connections where the wire from the battery
terminates at the starter.

Remove and clean the connections for the ground strap at the forward
transmission mount.

3. Possible failing starter solenoid.

I would first clean all connections and grounds (and see if it's cured).
  Then I would check the operation of the neutral safety switch
(automatics).  Then I would check the ignition switch and finally I
would check the starter solenoid.

Mike