You should have a short cover letter that explains why you want to work there and why you think you are qualified for the position; just put stuff that is especially pertinent to the particular job.
Make sure that your resume is easy to read. Look on the internet for examples. The employer isn't going to read paragraphs. Assume that the person getting it isn't going to read the entire thing - they are going to look for key points, such as the dates of employment, job title, and specific skills you have. So you want those things to stand out, like you might put your job title in bold, etc.
Make sure you are emailing it to the person who makes the decisions. You will get a better response if you know the specific name and email address of someone rather than sending just to human resources.
Follow it up in a few days with a phone call. Again try to talk to someone of importance rather than just a secretary. Just say you want to make sure they got your resume.
If there are a lot of people applying for the same position, you have to do a little extra so you get noticed.
Make sure that you show many ways to contact you: cell, home phone, email, etc.
