Before attempting to complete an application form, be sure to read it over. This is important for two reasons: 1) you want to put the information in the right place, and 2) you want to be sure you don't miss anything. Make sure that you complete every question and never say "see resume" unless the application form has very limited space, in which case you can fill in as much as possible and write "see resume for details".
Employers use application forms so they can locate information quickly (everyone's information is in the same spot). Don't make it inconvenient for them. Don't worry about repeating the information on your resume. In fact, copy the applicable information from your resume onto the application form. If the question does not apply to you, put a slash in the space or write 'not applicable' so the employer knows that you have read over the information and it does not apply to you. If you leave a section blank, it leads the employer to think that you missed the question.
One of the sections on the application form that people often fill out incorrectly is the work experience section. Even if you are including a resume, always complete this section in full. Describe your job duties in as much detail as the space allows. Use point form, and begin each point with a verb. This makes it easier for an employer to read, and less likely he or she will miss important information. Also, quantify wherever you can (i.e. Planned activities for five groups of 15 children aged seven to ten years; decreased customer waiting time by 22%). In this section, you should also answer the question in regards to your reason for leaving your places of employment (if asked). Here are some possible answers: