I've always played alignments a bit more casual than that. It's not a club you join or a cause you swear allegiance to, it's a description of the character's personality.
Paladins swear allegiance to a deity or cause, and bind themselves to that moral philosophy, but not all Lawful Good types do that.
As a DM I usually advise players when their actions are starting to reflect a change in their personality, and hence a potential change in alignment. One evil act won't make an LG character turn Evil, or even Neutral (though it can certainly ruin a Paladin's career). By the same token, one kind act doesn't make a Neutral or Evil character suddenly become Good. Alignment is a description of their over all personality, not a moment by moment barometer of their moral standing.
So in my games someone might slip in and out of any given alignment, one step at a time, as the game flows on and the character develops. Drastic changes, of course, reflect drastic events in a PC's life, so suddenly falling from LG to CE should be prompted by outside events, not someone suddenly deciding to turn Blackguard.