Probably not the best idea to let log files just grow and grow, but to instead use some sort of reporting system on the log which will cycle the logs and then extract and compress the data which is useful. In the case of Apache logs, something like webalizer will scan the log, pull out the data you need for a month, save it in it's own format (which has a huge size difference to straight text) and then truncate the original log file, ready to build up again.
