The best way to combat this is to find out your base angle, then match it throughout the tree.  What you have to do is find out what angle your base is at.  Assuming your trunk is circular, if you use the Oval Tool in MS Paint and draw and oval around the trunk that touches the sides and top and bottom, you get the dimensions of the oval, which tells you the angle of the base.  Once you have that, draw more ovals with the same height to width ratio and place them on your tree, this helps you maintain the same angle or ratio throughout.

I'm at work, otherwise I'd draw one up to give you an idea.  Hopefully you'll understand. :(