What a great bunch of answers.... I agree with all of them.
I originally worked on general (not relationship related) self-esteem issues in my early 20s in therapy (I'm now 45), and one thing that really helped me, though it sounds simple, was to locate myself properly on the spectrum of humanity -- to stop comparing myself to others in a really skewed and unrealistic and ever so f--ked up way. Everyone else was great, perfect, and I was just this little piece of dust, right? Oh my god, I was awful, so awful that if I was late paying a bill or lost a library book I felt roughly in the league of a murderer (thanks, crazy family upbringing).
My first step toward self-love was to start viewing others with more objectivity. Soon I began to see that despite my shortcomings and failures (which I had exaggerated beyond all belief, like I didn't get into an Ivy League college so I was scum, etc.; because of course the flip side of low self-esteem is a strange narcissism ) I did deserve to be included in humankind. In some areas I was average (which was okay!); in some I was below average, and in some I was clearly above average. Internalizing that lesson allowed me to feel a part of the brotherhood of man and not alienated or ashamed anymore. After that, I could feel regret at failure, but it could no longer make me feel unworthy.
I knew in my heart what I liked about myself, and I think we all do, though many of us are afraid to say it or own it.
Also, I admire Eleanor Roosevelt's advice especially as she was someone who began life with HORRIBLE self-esteem: "You must do the thing you think you cannot do." Challenging yourself by pushing your comfort zone and risking failure.
On the more superficial level, I firmly believe in buying new makeup and working out and getting manicures. It ALWAYS makes you feel better to invest in YOU.