What is the best way to avoid crocked side when bending on a bending iron?

Dear all,
I have come to ask for some advice again. I do my best to describe the problem I had..

I was bending some walnut sides last night.
No matter what I tried, the sides always become cocked at an angle.

This is my first attempt on bending a real sides for a ukulele. Maybe I just need for practice..?

I found an article from Stew Mac saying:
"Important tips....
Keep your work squared up! Keep the straight (top) edge of the sides aligned with the tip of the bending iron or with the bender’s mounting block. Don’t let your bending work become cocked at an angle. "
http://www.stewmac.com/freeinfo/I-0689.html

I have tried my best to do that, but the bended work is still cocked.
Can any one give me some advice on how to improve this?

THANK YOU !!
Jc


Edit: Sorry, I meant cocked not crocked in the title....

You might try lightly scoring pencil lines at the center of the bends for the upper bout, waist and lower bout. Make sure the line is facing up (on the convex side of each bend) and perpendicular to the soundboard edge of the workpiece. Use the lines as guides to keep the workpiece squared up when bending. Hope this helps.

It can be a problem to keep them straight. Just make sure you are standing square to the pipe and it is at a good height for working. Also, apply pressure evenly across the side. Sometimes I find I'm pressing harder with my fingers on one edge.
Just keep checking it often as you go. you can always touch it up if you're getting skewed