-Hanlon's razor - I always start from the assumption that people seek the happiness of others once their own basic needs are met, then go from there. Helps me avoid the "rich people/fanatics/foreigners/etc are trying to kill us all [because they're purely evil and nonhuman]" conspiracies.

-"What would happen if I apply x a huge amount of times?" - taking things to the absurd level help expose the trend and is one of my favourite heuristics. Yes, it ignores the middle of the function, but more often than not, the value at x->infinity is all that matters. And when it isn't, the middle tends to be obvious anyway.