The phrase "if I can help it" can be generally thought of as equivalent with "if possible", but there are some important distinctions. "If I can help it" is ultimately less forceful, meaning that the person is less dedicated to avoiding something.
For instance, a true vegetarian would never say, "I never eat meat if I can help it." The latter implies that there are situations in which the person feels obliged to eat meat (when Mom's cooking, when visiting a foreign country, etc.). A strict vegetarian, however, would deny such a situation exists (in everyday life, at least - the desert island question is another story).
The use of "if I can help it" shows us that the speaker believes that his/her influence over a given situation is limited. Someone willing to accept that is someone who is less than fully bound to their belief or desire. There are those who hold to something so strongly that they believe, barring perhaps threat of death, that they can always "help it".
In short, the user of the phrase "if I can help it" has a decided opinion but is not an ideologue.