Retry command until it is successful

I am writing a PowerShell script that stops an IIS Application Pool, does several other operations and then starts the AppPool again.

The problem is that the AppPool can take a long time to stop and if you try to start it before it is done stopping, you get the error

Start-WebAppPool : The service cannot accept control messages at this time. (Exception from HRESULT: 0x80070425)

Stop-WebAppPool -name $AppPool;
# do other stuff
Start-WebAppPool -name $AppPool;

What is the best way to get PowerShell to either

1. keep trying to start the AppPool or
2. force the AppPool to stop by killing the process.