How do you do a case insensitive search using a pattern modifier using less?
You can also type command -I while less is running. It toggles case sensitivity for searches.
You can also set the environment variable LESS
I use LESS=-Ri, so that I can pump colorized output from grep into it, and maintain the ANSI colour sequences.
Another little used feature of less that I found is starting it with +F as an argument (or hitting SHIFT+F while in less). This causes it to follow the file you've opened, in the same way that tail -f <file> will. Very handy if you're watching log files from an application, and are likely to want to page back up (if it's generating 100's of lines of logging every second, for instance).
Add-on to what @Juha said: Actually -i turns on Case-insensitive with SmartCasing, i.e if your search contains an uppercase letter, then the search will be case-sensitive, otherwise, it will be case-insensitive. Think of it as :set smartcase in Vim.
E.g.: with -i, a search for 'log' in 'Log,..' will match, whereas 'Log' in 'log,..' will not match.