There are a few basic life activities (eating, sleeping, etc.) that cannot be ignored and have to be maintained to some degree in order to function. Beyond these, however, it's remarkable how much variation is possible in what people care about and spend their time thinking about. Merely reflecting upon my own life, I can see how vastly the kinds of things I find interesting and important have changed. Some topics that used to matter so much to me are now essentially irrelevant except as whimsical amusements, while others that I had never even considered are now my top priorities.
The scary thing is just how easily and imperceptibly these sorts of shifts can happen. I've been amazed to observe how much small, seemingly trivial cues build up to have an enormous impact on the direction of one's concerns. The types of conversations I overhear, blog entries and papers and emails I read, people I interact with, and visual cues I see in my environment tend basically to determine what I think about during the day and, over the long run, what I spend my time and efforts doing. One can maintain a stated claim that "X is what I find overridingly important," but as a practical matter, it's nearly impossible to avoid the subtle influences of minor day-to-day cues that can distract from such ideals.
Needless to say, subtle cues are the reason advertising works so well. They're also why people are correct to warn against corrupting influences -- e.g., when fundamentalists discourage their children from attending liberal colleges. To an individual person, it feels so impossible that her concerns, attitudes, and emotional states could possibly change -- they just feel so right and necessary and inevitable -- but taking the outside view, even with respect to one's own life, clearly proves otherwise.
I have been fortunate that, for the past ten years or so, I've never lost an idealized commitment to the overriding goal of reducing suffering (though I have grown much wiser about how exactly that task is best attempted). Still, on a day-to-day basis, what I accomplish toward that goal has varied quite a bit; there have been days or even weeks when I've found myself completely distracted by other concerns. To some extent, this is necessary and important. For instance, those who take the approach of making money to prevent suffering need to find their alternate occupations intrinsically interesting or else will give up rather quickly. Creating subgoals with their own instrumental payoffs is essential for accomplishing any sort of long-term project. (Hence why people set deadlines, give homework assignments, schedule exams, perform periodic reviews, and so forth.) But from the standpoint of goal stability, it's also crucial not to let these subgoals take over and become ends-in-themselves. (For there's no intrinsic reason they couldn't be ends in themselves -- as I suggested at the beginning, pretty much anything apart from basic self-maintenance can become a person's chief concern.)
In view of the impact of subtle, everyday influences on one's unconscious mind, I recommend the following (to myself as much as to others): Don't just claim to care about reducing suffering in the abstract. Also manipulate your environmental influences toward the same end. Surround yourself with people who share that purpose. Read about altruism when you first wake up and are getting started with the day; Facebook makes this really easy these days. Cancel email subscriptions to irrelevant newsletters and add subscriptions on topics of which you want to be reminded regularly. Same for the blogs you read with RSS. Make regular time to remember why suffering matters -- for instance, by watching videos that depict the seriousness of suffering. Get a picture of a snake eating a live rat for your office wall. (These are just examples; YMMV.)
Even if you think these things are unnecessary ("Of course I care overridingly about suffering -- how could I feel otherwise?"), consider doing it anyway. Emotions can change like the wind, and one day's overriding concern is tomorrow's irrelevant cause.
Because these cues are so fickle and arbitrary, they rarely square with quantitative assessments of situations -- as evinced by, say, the tendency of many in rich countries to help moderately low-income people in their own neighborhoods over those completely destitute in the Third world, or the tendency of people to focus on human suffering even when that of other species preponderates by orders of magnitude. So it's important also to design one's environmental influences in a way that correctly represents the quantitative facts about a situation. Examples of this include hunger banquets or depictions like "If the world were a village of 100 people," in which quantitative data is translated into emotionally digestible form without losing its accuracy. It's for this reason that I hesitated to suggest the snake-eating-rat picture above, because in quantitative terms, the potential suffering of insects outweighs that of other animals by orders of magnitude in expected value. Similarly, it may be that recondite details of physics, cosmology, and anthropics imply vastly non-intuitive conclusions about the distribution of suffering in the multiverse and how best we can ameliorate it.
That's basically the theme of this blog. First do the math, and then come up with the "marketing" (feel-good images and unconscious persuasions) to back it up. But don't neglect the marketing: Math alone can't sustain motivation on a day-to-day basis. We also need the help of appropriately designed social and environmental surroundings to keep our emotions in line with our fundamental values.
[Edit from 12 April 2013:]
I want to qualify the tone of this post. It may have come across as suggesting that some activities are vastly important, while others are completely useless. Over time, I've come to see that the distribution of importance of different activities isn't as extreme as it may seem. Everything you do teaches you something, and sometimes it's the seemingly useless activities that lead you to try something new, as a result of which you "learn things you never knew you never knew." Like in a simulated annealing or multi-armed bandit algorithm, it's sometimes important to take random steps to avoid getting stuck in local maxima.
Also don't neglect the need to be generous to your own selfishness to avoid burnout or negative reinforcement with regard to altruistic activities. Fortunately, my experience is that altruism comes naturally as one of the most fun things I could do, in part because it feels so interesting and valuable, and in part because my friends are doing it too. Friends help you keep the seriousness of these issues alive.