What are the measures to keep your private key safe and in memory ?
Print your private keys to any form of paper that you can keep for safety. Don't trust your memory even you are the champion in memory verse challenges, it isn't enough to keep your funds.
You can also create a file on a notepad and save it to a flash drive and don't use that drive to any computer that you don't own. Don't use it to public places.
Are there any alternatives to retrieve the key back ?
If you lost it, that's lost forever.
You should have your back phrase in case you forgot your private key or password. You can also print your private key in any kind of paper than saving it in your notes. Just create a file where you can put all your password and private. Lastly, you should keep that file in a safe place so that you can avoid it from hackers.