(I'm thinking about my own personal philosophy these days and these are just some thoughts... responses appreciated!)
1) As humans we all exist, but it is our essence that makes us unique. What you are (essence) is the result of your choices (your existence) rather than the reverse. Essence is not destiny. You are what you make yourself to be. Our lives are not given to us, but must be developed consciously with care and consideration.
2) Living in the moment is essential, but we also interact and adapt based on our past experiences and future expectations. Yes, we are fundamentally time-bound beings, but we are also, much, much more than that. Unlike measurable (quantative), "clock" time, "lived" time is qualitative: the "not yet," the "already," and the "present" differ among themselves in meaning and value. We need to be aware of all of these. The impetus of living in the "moment" is that we should not let the past hang on us like a weight causing us to drown, or allow the possibility of an uncertain future to intimidate us to the point of inaction. Remember the lessons of the past, recognize the possibilities of the future, in order to fully live in the present.
3) Radical Humanism. Existentialism is a person-centered philosophy. It's focus is on the human individual's pursuit of identity and meaning amidst the social and economic pressures of mass society for superficiality and conformism. It is our responsibility, as free and conscious beings, to create meaning out of life and to develop an authentic essense. It is also, in my opinion, in this regard, our duty to help others develop their response-ability to do the same (for me as a teacher this is the core of an existentialist pedagogy). In this we are cultivating free, ethical and responsible individuals who care about their community. My radical humanism does not discount other beings in this world... it is holistic, in the sense of recognizing that humans are just one set of beings that live and share in the development and continuation of the broader ecosphere.
4) Freedom = Responsibility. Existentialism is a philosophy of freedom. It requires that we step back and reflect/reassess on what we have been doing and what effect our thoughts/actions have on the world. In this sense we are more than just individuals, we are members of larger collectives and our personal ethics always extend beyond ourselves (existentialism is not vulgar egotism). In this we can only be as "responsible" as we are "free." Response-ability, the ability for people to respond to the problems of their society and the impetus for them to care beyond themselves, is only realized by free, authentic and ethical beings. Where there is mindless conformism, shallow consumerism, or brutal oppression, you will see a breakdown in the development of response-ability (both in the ruled and rulers... or, manipulated and manipulators).
5) Ethical considerations are the primary questions. We all understand ethics and freedom differently, this is a given, and we must bring each of our understandings into play and sharpen our ideas through open/free public discourse. In this we, as individuals, as a community, as a society, and as a global ecosystem, should consider ethical questions. Each individual is responsible to develop and consider the authenticity of their own personal lives and their society.
Existentialism is a philosophy of living authentically in the world, but in the realization of our authentic self we also have an ethical responsibility to ensure that others have that same opportunity. My authenticity should not be at the expense of your opportunity to realize yourself (for example, we are not bloated ticks that feed off the misery of others in order to realize some twisted sense of self).