Here are some tips to show you the basics of the configuration archive function on Cisco IOS

You can setup you router to store all older versions of your configuration, like this :

  log config
  record rc
  logging enable
  notify syslog contenttype plaintext
  path flash:startup-config
  maximum 14
  rollback filter adaptive

This will, every time you write memory, copy your old startup-config to a file with the current date, like this :

16 -rw- 7391 Mar 20 2012 20:30:04 +02:00 startup-configMar-20-18-30-05.366-52
17 -rw- 7391 Mar 21 2012 16:42:50 +02:00 startup-configMar-21-14-42-50.316-53

You can see the current in-memory archives :

> sh archive
The maximum archive configurations allowed is 14.
There are currently 3 archive configurations saved.
The next archive file will be named flash:startup-config--3
Archive Name
1 flash:startup-configNov-15-20-28-19.132-0
2 flash:startup-configNov-16-07-39-50.027-1
3 flash:startup-configNov-24-15-32-20.014-2