Product review (5 stars): Inexpensive Last Resort For Old Compressors
I suspect HVAC techs would advise caution (I would as well). But installing one of these is a relatively quick and effective fix for someone comfortable with household electrical work, 120/240v, circuit breakers, disconnecting power to the compressor - and especially - how to safely discharge a large capacitor to avoid a potentially life-threatening shock.

If you're ok with that, disconnect power to the compressor, open the panel, discharge the capacitor, unscrew the bracket holding the cap - the same bracket you'll clip the hard start to, connect one lead to the C terminal on the capacitor, the other to the Herm terminal, reattach the bracket and put the panel back. Connect compressor power, go inside and and start it using the thermostat.

You won't avoid the inevitable using one of these - and they don't always work depending on what else is happening with the compressor.
But if you're stuck mid-season with an old compressor that won't kick in a hard start might get you through the remaining hot weeks and buy you time before shelling out for a replacement.