to best protect your work with MyEtherWallet you can use it :

a) Locally, that is, on your computer, without entering keys and passwords online;

b) as a cold wallet, on a computer that is not connected to the network at all.

The second option is most preferable because it provides the highest level of security. Of course, the use of a cold or local wallet does not eliminate the need to install anti-viruses, and maintain a neat lifestyle on the Internet as a whole.