Recruiting is all about minimizing risk. Essentially, recruiting means a person compiles a list of strangers and systematically whittles it down to a point where they can recommend a few select candidates for a particular position. They've never personally worked with these people before, so they rely on tools and processes such as resumes, cover letters, screening interviews, full interviews, second and third interviews, references, etc. to get the list down and find the best candidates. They obviously want to minimize the risk of a bad selection.
So how do you, as a job-seeker, help the recruiter to minimize their risk?
Offer as much "proof" as possible that you have the experience, skill, and expertise to do the job, and that you are the most likely to be successful in this new role. Here are some ways to do that:
Tangible Achievements on Your Resume
Resumes need to include achievements in order to show that you've been successful, but these achievements need to be tangible as well because they come across as much more believable. Compare the following two points and see which one sounds more believable to you:
Achieved a significant improvement in customer service.
Drove customer satisfaction scores from 61% to 84% in the first year.
Of course it's possible that the second person is lying about their scores (don't ever do that - it will come back to haunt you), but on a resume it comes across as much more trustworthy, and building trust is key.
Awards
Awards can be very powerful examples of your skills and talents - in fact, I think it's terrible that many resume templates have a small section for awards at the very bottom (just another reason to never use templates). They're much more important than that because they provide the recruiter with tangible "proof" of how well you performed vs. your peers. It's not just YOU saying that you were better than everyone else - an award shows that your company has actually said it. That carries much more weight.
If you've won awards, of course list them on your resume and provide context for the reader but, if you can, go further - if you have an actual certificate or trophy, add a picture of it to your portfolio or LinkedIn profile. Any time you can provide proof, do so - it helps build credibility.
Letters of Recommendation
Reference letters are a bit outdated now but they can still be helpful. Typically they have been replaced by LinkedIn recommendations (see below), but some people still prefer to write an actual letter or email rather than posting something online for everyone to see. Either way, they are valuable because they provide the recruiter with an objective, third-party assessment of your abilities. Rather than just you saying how skilled and successful you are, they can rely on someone else saying it. Again, it minimizes risk.
LinkedIn Recommendations
Recommendations on LinkedIn have become the new letters of reference - only they're much better. Not only can recruiters see them right away on your profile, they can also check out the other person's profile to confirm their position with the company. In the old days, recruiters may have had concerns about fake letters of recommendation being typed up in someone's basement, but now a scam artist would need to fake another person's whole LinkedIn profile and then have that "person" give them a recommendation. (Now that I think about it, I'm sure someone out there has done that.)
At any rate, LinkedIn recommendations have become almost mandatory for any serious job search. If you don't have any, get some soon. Recruiters will wonder why if you don't.
References
Although LinkedIn recommendations are great, recruiters will often still want to talk with someone on the phone who can vouch for you, so make a list and have it ready for interviews. These may be people who have also given you a written letter or a LinkedIn recommendation, but they don't have to be - in fact, the more the merrier. At a minimum, have a list handy with 3-5 professional references on it, along with their contact info.
Performance Evaluations
Past written performance reviews can be a great way to show future employers that you've been successful. Why? Because the information provided doesn't just come from you - it comes from someone higher up who is assessing your performance and outlining your strengths. Therefore, they are much more objective than you can ever be about judging your talents and success, and recruiters would prefer objectivity.
If you have copies of positive performance reviews, consider taking them along and sharing them with the interviewer (IF they don't conflict with a confidentiality agreement). It might be helpful.
Customer Appreciation Letters
Letters or emails from customers, clients, or anyone else who has praised you in some way can be very helpful. Keep these and bring them to interviews to show recruiters. Again, they offer a third-party perspective that will help you build trust with the recruiter.
Portfolios
A modern-day portfolio can really be any tool that you use to provide recruiters with concrete examples of you work, and other proof of your success. You can take a professional binder to an interview, you can create a website or slideshow online, or you can simply upload samples of your work and success to your LinkedIn profile (if you're okay with everyone seeing them). In fact, many of the items above, such as evaluations and letters, could be included in a portfolio - just be cautious about privacy and confidentiality.
Your resume should do everything possible to showcase your talents, build trust, and minimize risk for the recruiter, but the resume is also just one point in a job search. Throughout the process - from initial contact to interviews and references - think about ways you can prove to the recruiter that you're a perfect fit. Don't ask them to just take your word for it. Offer proof whenever possible.
I hope this helps and I wish you all the best in your job search!