There are a number of applications that open a log file and just keep writing to it. This is, of course, a problem when you rotate logs as the application just keeps writing to the renamed log instead of opening the new one.
In logrotate you can handle this with the
copytruncate option. It will copy
the contents of the log to a new file and truncate the old one. Your
application will merrily continue appending to the newly 0-sized log.
If you want to do that manually, consider the truncate command:
truncate --size=0 /var/log/foo.log