For an entire month, write something you like about yourself every single day. From your ability to write well or how much you like your eye color, any compliments that's truly positive counts. At the end of the month, go through your positive thoughts and reflect on them.
Low self-esteem is often a problem of putting too much emphasis on things that can't be changed or that are too far in the future to tell the outcome. I'm not talented enough, I'm going to fail - stop thinking of these thoughts and live in the now.
When you write down your your feelings, you get them out in a heathy way. Once you release them through quick and freeform writing, close your journal, for the day and don't open it again until it's time to write again.
When you catch yourself being negative, actively acknowledge it. When you recognize your negativity, you give yourself the chance to response with positivity.
"If someone else was in my position, would I make them feel negatively?" Sometimes looking objectively at yourself and your situation can make you feel more positive.
Instead of falling back on verbal onslaughts with yourself, start talking with yourself like an adult. Converse with yourself. Why are you feeling like you do? How can you make it better?
Normally those with low self-esteem are also very negative in other areas of their life. If you find critizing others, stop this practice. Be more positive in all areas of your life.
Doing something relaxing can help you become more positive with yourself. Try and let go of all your inner anxiety and breathe for 15 minutes a day - or more.
Don't be a backseat driver. When you let your emotions run your life, you aren't truly in control. Start actively participating in your life to help develop a more positive attitude.