I have noticed that a two line answer that anyone can come up with usually has more upvotes than an answer that is more difficult to come up with. Is this trend simply a result of pure laziness as people are less likely to read a longer answer? What can be done to reverse this unfortunate trend?
I wouldn't really bother too much about the unfairness of up/downvotes. It is true that the number of upvotes I receive for my solutions is often (though not always) a decreasing function of the time and effort I spent on the question. However, the questions that attract my attention are about uniformly distributed in difficulty on a scale from $0$ to $1$ and, given that $\int_0^1 x\,dx=\int_0^1 (1-x)\,dx$, I do not think that any "crusade for fair voting" will change my or anyone else's score dramatically.
I am not too much bothered with this, after all the goal of our activity here is not to "earn points'. The trend you noticed applies not only to MO: I notice that citation of my published papers is also a decreasing function of the efforts spent and probably this applies to many mathematicians. Another trend is that "soft question" usually get more attention, and more up votes than true mathematical questions. It is also easy to explain more people would be interested in a soft question, and more people will understand it.
I think these trends are inevitable, and we should not fight them. If someone really wants many points, s/he may consider asking many good soft questions:-)
While I don't want to endorse the suggestion in Joseph van Name's second sentence, it does seem that answers which are not quite to the point, but "sound good", can get a slew of upvotes. I am currently looking at one highly upvoted answer which seems to me incomplete, in the sense that I think some of its arguments need a bit more justification. It's of course possible that I am being particularly dense today and that the argument is correct, but I think its author has definitely overlooked some details.
As to what one can do about it, the only possible thing is to leave a comment on the answer, downvote if one feels that is appropriate, and so on. One can't go round badgering individuals to rescind upvotes (as in life, so on MO) but one can try to explain one's case and see what happens as time goes by.