system logs management

[John <joney@clara.net>]

What steps do users take to keep logs down to
reasonable sizes?  I have seen nothing on this aspect
of management and so have been using 'rm' and then
creating a new file.

If it is possible, I would like to remove everything 
older than say seven days on a twice monthly basis
after checking there is nothing I would like to archive
for possible future reference.

I haven't yet found a command to empty a file as
opposed to removing it - does one exist?

Grateful for any help or pointers.

Regards,
John.