Looking for a new job can be stressful. Just putting together a resume and double checking all your stats and past experiences for accuracy. One of the biggest ways to get a good impression on a prospective employer is through reference letters. These are letters written by people who can vouch for your work, skills, and personal characteristics. Getting trusted individuals to become one of your references can be crucial to getting a brand new job.
Being on good terms with the person whom you'd like to ask for a reference is a must as their opinion is going to be responsible to get you hired. While a reference certainly shouldn't lie in favor of a prospective employee, they shouldn't lie against one either.
Make sure that the person is a qualified reference. In other words, they should have direct knowledge of your work and/or your personality traits such as work ethic.
If you aren't a personal friend of your target reference, then a formal letter may be a great way to ask. This is very business-like and a good way to go about asking a higher up boss or executive who is familiar with your work.
More personal friends or co-workers can be asked in a variety of ways. The comfort of being closer personally allows more informal types of communication such as email or social media.
Avoid writing for the reference if possible. A more personal account is needed, particularly if they're interviewed.