How do I comment lines in .plist file?

Syntax of plist is pretty much like XML, so you can use XML style comments.

<!--
<dict>
    <key>Hello</key>
    <string>World</string>
</dict> 
-->

Xcode seems to believe that the <comment> tag corrupts the data. It will also strip out XML-style <!-- comments --> when viewing the plist in Xcode. It seems that a string entry, e.g.

<key>Comment</key>
<string>My comment</string> 

is the only way to reliably persist the comments.