I read Little Light's post on dangerous (hopeful) thinking today, and found it resonating strongly with my own attitudes.
The activism I try to embody is positive: it seeks solutions, a world that we want to see. This is one of the things that I like about Climate Camp: it's not just about protesting and trying to stop bad things, it's about trying to build good things. Providing alternatives; sharing skills and knowledge; building communities. I take part in protest-oriented direct action, but I put more energy into promoting cycling, trying to get better at growing my own food, finding carbon-cheap ways to operate my life (and then promoting those). For me, being the change you want to see in the world is vitally important.
Having said that, the comments at the bottom by shah8 are also important: if we think only of the end and not of the means (or of how the end is operated), then we risk simply ending up with another set of wrongs to replace the ones we have. For me, the way to avoid this is to treat your hopes and aims holistically. It's not OK to achieve one thing by sacrificing something else important.
Of course, the problem then becomes: are all your aims compatible? Can they be treated holistically? If not, do you need to rethink them so that they can? Sometimes it is legitimate to sacrifice one thing for another. I'd love for it to be possible for everyone in the world to have the same range of choice and luxury as those who are well-off in the developed world, without causing poverty and environmental destruction. That's not possible, so instead what I want to move towards is everyone being able to survive with some level of comfort, in a sustainable and non-poverty-generating way. (Which is of course an unpopular concept in the developed world, entailing as it does a lower standard of living for us. Myself included.)
From a sustainability point of view: striving for a positive goal is arguably more personally sustainable in the long term than simply fighting a negative. Sometimes it is necessary to put energy into fighting a negative; but having a positive vision of what you want to see instead, and trying to include at least some positive movement in your negative action (community-building is always a good one), will tend to help you hang on in there for the long haul.