Always include a cover letter
You may think that your resume says all you can say about yourself. In fact it just gives us the basics – tell us about YOU and what you want. Firing off a resume without taking the time to write a personalised cover letter can make recruiters think you are randomly applying for roles and not properly invested in the current role. You don't want to wait until the last minute to put your application in, but you also don't need to apply within 5 minutes of the ad being posted – take the time to write a proper cover letter. Always make sure your cover letter is a separate attachment, not the body of an email or in a 'type your cover letter here' section of an application website. This gives you the control as to what it looks like.
Do Your Research
A good cover letter will always be tailored to the specific position. Look up the website of the company or recruitment firm, see if you can find out who will be accepting the application and address it to them. Specifically mention the job title and company (if you know it) and make sure you mention how you specifically can fulfil the role.
Don't just talk about what you've done – talk about what that achieved. Showing your contribution to your previous employers will let potential employers see what you can contribute to them.
Include your interests on your resume – it gives us a better idea of you as a well rounded person. Do you do volunteer work? Enjoy playing a particular sport? It will help us know if you are a cultural fit for the role.
Review, Review, Review
Re-read your application. Then do it again. Does it say what you want it to? Is there any additional information that could be relevant? Is the spelling correct? Is your grammar correct? With a large number of applicants for jobs at the moment, the smallest error could be the difference. Make sure you check everything before sending your application out.
A lot of candidates send their applications in written in a word processing document. While this is where you will be writing your application, you should always convert the file to a PDF. Most word processing programs allow you to do this without needing to purchase an additional program. By converting to PDF, it means that no matter what type of computer the prospective employer has, how it looks on your screen is how it will look on theirs. If you keep it in a word processing file, you run the risk of mark-ups to show (ie changes you have made) or the font or alignment to be out of whack. It also looks much more professional!