Mine was doing the same thing. About once a month it wouldn't crank. After a month or two, it did it twice in 1 week before I had to go on a 8 hour trip. New starter and relay. Don't really know what one it was, but it's been good since.
I would check the wires around the starter for corrosion and make sure they are tight.
When it's not starting, you would have to see if you have power at BOTH the big terminals on the relay. (the one bolted to the passenger side fender)
Or if you can't watch the meter and turn the key at the same time, you can put power across the 2 big terminals, if you KNOW power is getting through the relay and it still isn't cranking, problem is at the starter.
It could be related to the ignition switch or trans too. When you turn the key "on" and the lights come on, do they go back off/on when you turn the key to "start"? Ignition is fine if they do.
If you have an auto, when it doesn't crank, with the key on, foot on brake, hold the shift lever up into park, or try in neutral.