I use a strip stick. You can probably make your own with half round and some thin batting and muslin. I bought mine at a quilt show though. It shows them pressing the seams open but with a little practice you can also use it to press seams to one side and they come out nice and straight. When you use it to press seams open you do it from the back side and when you use it to press seams to one side you do it from the right side.
http://thestripstick.com/
Everybody else is going to tell you you have to sew them from opposite ends and that'll solve your problem. It might help some but the real problem is in the pressing.