Starter solanoid, worn starter brushes, neutral transmission switch possibilities as well. Read the archives.
When a car developes an intermitent starter action the first thing I check is if the starter solinoid is getting a good 12v supply. Then activate the solinoid manually. If I still have a good supply voltage to the solinoid at this point and no starter action the problem is generally localised.
The next time it will not start tap the starter moderatly hard. If the key rotates the starter right after that take the starter off and test. Possibly worn brushes or marginal soilinoid. These are the most common senarios really with an intermitent starter.
On an older car yes bad or oxidized connections are an increasing possibility as well of course. Yet I tend to eliminate the starter from the general equation first of all. Since it is usually the most likely culprit. Eliminate it first.