My usual reply to this question - It wasn't as hard as I thought, but, by no means was it easy.
Advise - study, study. Especially the Certification Manual for Welding Inspectors. Make copies of the practice test so you don't mark all in the books. Read a chapter then immediately take the test and grade yourself. Go back and read where your weak points are. After you finished the book and tests, wait a day or so and then take all of the tests at one time. Read the book at least a few times.
At seminar time, they hand out tons of practice tests for all three tests. DO THEM, and STUDY, STUDY, STUDY the week you're there. If you're doing the 1104 tests, make sure you know the two annexes, no matter what you instructor says (mine was mis-informed about the added questions).
Other than that - STUDY!!
I went to school for QC/NDT, so I was familiar with a lot of the subjects. I didn't have much time to study, so I had to make very good use of my self "crash course", and the seminar sure helped quite a bit.