How does one help someone gain self esteem?

i try very hard to compliment my significant other constantly, but they have been bombarded all of their lives with silent and verbal criticism from family and friends, and i often hear the phrase, "I'm glad YOU think I'm pretty, but i just can't believe it for myself." I want so badly for them to feel good about themselves from the inside instead of needing constant external validation. Is there anything i can do to help them see how amazing they are?