There are so many tips you can google, some are very complicated, some are useless.
Now I want to share my experience to reduce memory usage of Apache on Centos server.
You have to have Webmin installed on your server first.
Server >> Apache Webserver >> Global configuration >> Processes and Limits
There are three figures you can change to reduce the memory usage.
Maximum spare server processes: 20 (default), I changed it to 10
Minimum spare server processes: 5 (default), I changed it to 4
Initial server processes: 8 (default), I changed it to 5.
The total memory usage reduced from about 500M to 300M.
Every httpd occupied 20M roughly. So I cut 10 server processes, is to save about 200M memory.