You want to ensure that you have sufficient RAM in your server.
Find the instances of Apache in your process list, and determine an average memory footprint for an Apache process. Multiply this number by your peak load (maximum number of concurrent web clients you'll be serving).
Because there is very little else that you can do at the hardware level to make your server faster, short of purchasing faster hardware, it is important to make sure that you have as much RAM as you need.
Determining how much memory you need is an inexact science, to say the least. In order to take an educated guess, you need to observe your server under load, and see how much memory it is using.
The amount of memory used by one Apache process will vary greatly from one server to another, based on what modules you have installed and what the server is being called upon to do. Only by looking at your own server can you get an accurate estimate of what this quantity is for your particular situation.
Tools such as top and ps may be used to examine your process list and determine the size of processes. The server-status handler, provided by mod_status, may be used to determine the total number of Apache processes running at a given time.
If, for example, you determine that your Apache processes are using 4 MB of memory each, and under peak load, you find that you are running 125 Apache processes, then you will need, at a bare minimum, 500 MB of RAM in the server to handle this peak load. Remember that memory is also needed for the operating system, and any other applications and services that are running on the system, in addition to Apache, and so in reality you will need more than this amount.
If, on the other hand, you are unable to add more memory to the server, for whatever reason, you can use the same technique to figure out the maximum number of child processes that you are capable of serving at any one time, and use the MaxClients directive to limit Apache to that many processes: