Humility

Rav Moshe Kordevero, in Tomer Devorah (Perek 2) says, that humility is the key to all good Middos (character traits).

Humility he says means, considering yourself "Ayin", nothing. Once you achieve this, all other Middos will fall into place automatically.
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This is not easy to achieve, as we all suffer from the disease of ego to a degree.

The Ramak suggests five things, that can help in this area:

1. Run away from Kavod
If people shower you with honor, you will get used to it and crave it. Then it will be very hard to extricate yourself from gaava (vanity). You must run from Kavod at all costs.

2. Focus on Your Weaknesses
Just because people honor you, for what they think makes you great, you still know the truth. Don't delude yourself, just because the people are mistaken. Be honest with yourself, and know the truth about all your shortcomings. If they knew these things, would they still honor you? Probably not, so get over it.

3. Appreciate Disgrace
Think about all your aveiros, and what you really deserve from Hashem. When you are humiliated or disgraced, be happy. It is a lot easier way to be punished than, getting sick, losing your money, or having someone close to you die. Better to suffer shame, than real problems.

The Ramak then brings two more thoughts that he says will help, but are not as effective as the first three.

4. Respect All Creations
If you embarrass even the smallest most insignificant creation, it is a disgrace to its Creator, Hashem. If Hashem created something he must be very pleased with it or he would have done it. How dare a person belittle it.

5. Fill Your Heart With Love of Every Man
Even love Reshaim, daven that they should do Teshuvah. When you see a disheveled vagrant, rather than loathe him, ask yourself what is the difference between him and I, a little money?
Don't look at people's bad traits, only their good ones.