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.
Yes printing your keys on a paper will be the best solution rather than to save it on a flash drive. Why am i saying this, last month while i was in my room working thieves broke into my room and toke my laptop when the drive was connected to the laptop. Now i have lost everything, my little dollars is gone.