I can think of three options to reduce the logfile size.
- Make the logs smaller my gzipping them. AFAIK this is a syslogd option.
- Only log things that may be useful. The cleanest way is to eliminate the request you know are not useful (like images, css, js etc.) based on their full URL (This way you keep the unexpected hack attempts).
- Make the Apache only log a subset of the traffic.
One possible way of doing this is the conditional logging you mentioned. Now conditional logging uses the SetEnvIf Apache feature. The actual syntax specs of SetEnvIf state:
SetEnvIf attribute regex [!]env-variable[=value] [[!]env-variable[=value]] ...
So how about using this to make an expression that only matches the 'even' (or 'odd') IP addresses of the Remote_Addr?
You can cut it even further by limiting the IP ranges even further.
Of course you could also look at the reason for your question here: What makes the logfiles 'too big' and 'unmanageable'? What information to they hold for you?