Good friends can improve your health and even promote a longer and fuller life, according to “The Australian Longitudinal Study of Ageing,” produced by Flinders University Centre for Aging Studies. Letting your best friend know you love her can cement the relationship, so that it stands the test of time, and make it more rewarding for both of you.

Encourage Her

Everyone needs encouragement, especially during the rough spots, and it means a lot coming from someone who loves you. A close relationship with your best friend can give you the resilience to make it through adversity, according to the PBS article "What is Resilience?" Promise your friend your support, especially when she needs a shoulder to cry on or someone to remind her how strong she is. Drop a note in her locker expressing your love and appreciation. Send her a loving text on days you know are especially challenging, such as when she’s facing a test or when she breaks up with her boyfriend. Give her a hug, kiss on the cheek, high-five or a quick back rub.

Share Time

Your closest friend is the person you see and talk to almost every day, according to psychology professor Margarita Azmitia in "R U Friends 4 Real?" for the American Psychological Association. Love strengthens through time spent together. Take a walk in the park to look at the flowers and share a picnic lunch. Let her pick the movie or your dinner destination for the evening. Window-shop, even if money is tight, and talk about what you would like to buy. Double date or share your dreams for the future. Laugh together over jokes or enjoy a book together.

Give a Gift

Small gifts between friends communicate love and the knowledge that someone important to you is thinking about you. Pick a flower for her or grab her favorite chocolate when you’re shopping. Put together a picture album, digital or prints, of your times together with a brief note about each picture. Play her favorite music when you’re together or send her a musical tribute via your favorite radio station. Get tickets to see her favorite entertainer and see it together. Give her half of a Mizpah charm, which contains the message “Watch between you and me when we’re apart," and both of you carry your half.

Lend a Hand

When push comes to shove, you count on your best friend to lend a hand, and you do the same for her. She needs cookies baked for her sister's birthday -- you're there. She needs her end-of-year paper proofread and critiqued, and you, as an honest and caring friend, are there to provide constructive criticism. Whether or not you express the sentiment with words, your willingness to help will show her you love her.