By picking good algorithms, implementing them carefully, and making sure that your program isn't doing any extra work. For example, the most microoptimized character-copying loop in the world will be beat by code which avoids having to copy characters at all.

When worrying about efficiency, it's important to keep several things in perspective. First of all, although efficiency is an enormously popular topic, it is not always as important as people tend to think it is. Most of the code in most programs is not time-critical. When code is not time-critical, it is usually more important that it be written clearly and portably than that it be written maximally efficiently. (Remember that computers are very, very fast, and that seemingly ``inefficient'' code may be quite efficiently compilable, and run without apparent delay.)

It is notoriously difficult to predict what the ``hot spots'' in a program will be. When efficiency is a concern, it is important to use profiling software to determine which parts of the program deserve attention. Often, actual computation time is swamped by peripheral tasks such as I/O and memory allocation, which can be sped up by using buffering and caching techniques.