also, what made you believe the earth is flat?
I just created a thread for this question not too long ago.
http://theflatearthsociety.org/forum/index.php?topic=29417.0Basically, people said it looked flat. Of course, with geometry, you can calculate the curvature of the horizon with any given altitude, and since the size of the earth is so large, you would have to go very, very high to see a distinct curvature. Based on what you see out your window, you can't draw any conclusions, or even a decent hypothesis, about whether the earth is flat or round.
Also, some people said because they don't like the theory for how gravity works, even though the force causing their upwards acceleration has even less evidence.
I think some people might have mentioned Rowbotham's experiments, but they are flawed since they can only be replicated under conditions which produce a superior mirage so you can see over the horizon.