There are different ways to cover gaps in your resume. There are many reasons why these gaps may occur. There are layoffs, family emergencies, health issues, educational leaves, and likely other things I haven't covered. With the current climate in the business world, gaps of 3 months or longer are not as rare as they used to be.
One way to address gaps is to use the years of employment instead of month and year. If you spent more than a year at a company, just use years for the employment period to not draw attention to the gaps. For example, you can list 2006 - 2008 instead of March 2006 - August 2008. I still believe that the most important parts of one's resume are the summary and skills section at the top.
If you were involved in any unpaid activity such as volunteering or consulting during your period of unemployment, it is legitimate to list this on your resume for the employment period. Any work that maintains or enhances your skills will work to your benefit.
If the gap in your resume occurred a long time ago, don't include it. Your resume needs only to cover the last 10 years of employment and no more than 2 pages.
In the situation where one returns to school, include it in the resume. This is an acceptable gap to have. You may also want to explain this in your cover letter. A simple sentence or two should be adequate. In the example of the individual I was contacted about, he is on target with identifying how the education he received, while not directly pertaining to the field he wishes to return to, helped him improve in skills that he can directly apply to the work that fits him better. There is also no reason why he should exclude the land surveying position from his resume. What he should do is use the experience there to show accomplishments and skills that will help me with the position he is now seeking.
A final solution would be to consider using a functional resume. With some recruiters, this can be a warning flag, but it does allow you to accentuate the skills and abilities that you have. If someone is concerned about any gaps, they should address them generally in the cover letter. Being truthful and honest are key strengths that will help in virtually any job one is seeking. If it comes up in the interview, explain it in general terms but don't let it derail you. Maintain your focus and get the job you want!

