My method may seem narcissistic: I look myself in the mirror when I can. It helps me realize that I'm not as ugly that others mention, I'm unique in my own way and I have to learn on how to love myself first before loving someone else. I'm not perfect, but then again no one in flesh really is. I have my ups and downs, but I know I'm worth the time. I just have to repeat to myself these things when I doubt myself. It's an unfortunate struggle as a result of allowing myself to be hurt, instead of standing up and be courageous.
If you have someone with low self-esteem, don't pity. Help them by supporting them and giving them words of compassion AND motivation. ;)