You're on the right track by putting your variables in the "data" and "idata" segments whenever possible.

Some other hints:

• Play around a little bit by writing the same program in different ways. Look at the generated code to see what works best with your compiler. For example, you can write an endless loop in lots of different ways:
  Save This Code

  while (1) { /* Loop body here */ }
  for (;;) { /* Loop body here */ }
  do { /* Loop body here */ } while (1); 

  or even (*gasp*)
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  LoopTop:
  /* Loop body here */
  goto LoopTop;

  Try them all and see which generates the best code. Then do similar experiments for other constructs that you use frequently. Eventually you will get a gut feel for how your compiler works.
• Use the smallest data type that will do the job. In other words, don't use an 'int' if all you need is a 'char'.
• Similarly, don't use floating point arithmetic if you can somehow use integers instead.

Others will have many more hints, I'm sure.

One more thing ... as in most things, there are tradeoffs here. Don't worry so much about "efficiency" that you forget other important things like maintainability and readability and portability. Execution speed can be important, but it is usually so only in a few "hot spots" within any given program. Reserve your tricks for those places (assembly language might be the best trick, by the way), and use your energy elsewhere to make your programs easy to read and maintain.

-- Russ