Disclaimer: if you don't understand automotive wiring, especially starters, don't do this. These instructions are for experienced mechanics who have common sense. Also, your car might be different. You could very easily cause a fire or explode your battery.
One day, I turned the key and the starter didn't turn. No amount of wiggling would do the trick. The gauges ticked, so some current was going to the starter solenoid, just not enough.
Given the warning signs of having to turn the ignition key just right to get the car to start, weeks ago I bought a Borg Warner starter relay for a Ford. $13.
Parts: large gauge crimp-on terminals (usually from high amperage auto audio places), heat shrink tubing, electrical tape, selection of ring terminals, large sheet metal screws, fine threaded standard nut to fit the "S" terminal on relay (purchase at same time as relay, or visit the hardware store before starting the installation).
Disconnect the battery ground terminal. I screwed the relay (it has mounting holes) to the rear of the pan on the side opposite the battery. Cut the fat wire between the alternator (generator) and battery. Crimp large ring terminals on each wire, and attach both to the large terminal on one site of the relay (Number Two). Cut the wire to the starter. On my car this goes through the rear of the pan right near where I installed the relay. Put a ring terminal on the starter side, and attach this to the large relay terminal OPPOSITE the battery-alternator wire (Number One). Put a small ring terminal on the old starter wire running back from the ignition switch. You just cut this in the previous step. Connect to the small "S" terminal on the relay (Number 3). You'll need a fine thread standard nut. The nut on the relay holds the terminal in. You can remove this, but you're better off to use a new nut. The stock Ford connector is actually a push on and doesn't use a nut. Remember to buy an extra nut or two when you buy the relay.