The best position for going into a trick is to have your handelbars perpendiculaire with the frame, a reletivly tight grip, standing with you knees slightly bent. You want your feet to be even with the ground, with one foot to the front. You want your weight retivley centered, however for some tricks it might have to be more the the front back or sides. Here is a good picture that demonstrates:
http://z.about.com/d/mountainbike/1/5/H/coast_280.jpg
The picture is on a mountain bike, but the same position is good for bmxing too.