To check the battery:
Check the voltage before you start in the garage, should be about 12V
Now start the car and check the voltage, should be about 13V
Now do your usual run and shut down. Check the battery voltage if she wont start.
Now check all the connections from the battery to the starter. Disconnect the battery, and remove each conection, and clean with electronics cleaner, wire brush off all corrosion, and reinstal and seal with electronic sealer. Do the same cleaning/sealing to all connections to generator, and ignition switch.
If this does not cure your problem try this:
Might want to make sure you add a relay from the ignition switch to the solenoid.
I think the ignition switch gets corroded over time, and does not provide enough amperage(current) to the solenoid to engage the starter. I had the same problem with my 78 spider, and this solved my problem. Also you might want to lube the solenoid to make sure it moves smothely/freely.
Here is a really nice write up from the Kansas City chapter of AROC:
http://www.geocities.com/MotorCity/...7/spicrela.html
For some reason people on this board dont like to add a relay to their starting system, all modern cars have these relays to solve the voltage/current droop thru the ignition switch, so I dont know why nobody here want to do it.
It sure is a lot cheaper than rebuilding starters/alternators/ignition switches, etc.