What is your definition of centering the channel? Is it just that the tire bead is not sitting on the upper shelf? Because that by itself isn't enough in some cases.

The biggest trick I've learned is that you have to also pull the slack out. I may totally fumble how to describe this with words...but here goes.

1. Set the bead on 60-70% of the tire. This should be no problem.
2. The remaining 30-40%, face the wheel so that the unseated portion represents 10:00 to 2:00 on a clock where 6:00 is pointing to your waist.
3. With your right hand, hold the top of the wheel at about the 1:00 position and pull inwards to your waist.
4. Now use your left hand and pinch the tire starting from 6:30 and pull it towards 9:00.
5. Without fully letting go of the tire as you pinch, work your fingers towards 9:00 where you will now be pulling towards 12:00. Essentially what you're doing here is pulling the tire beads up against the channel removing all the slack. You should then notice that the section that was unseated at 10:00 can now easily seat to 11:00.
6. Repeat the same process now by using your left hand to hold at 11:00 (while pulling the wheel towards your waist), and your right hand pinches and works the tire from 5:30 back towards 3:00, and back up to 12:00. Again, you should feel that the section of tire that was only seated up to 2:00 can now easily get to 1:00
7. From here on, you generally have so much of the tire on that the last section from 11:00 to 1:00 can just pop on with thumbs. But if not, repeat the process on one or both sides to pull just that little bit extra slack.

It's really this process that makes all the difference in the world. I've had some funny moments where guys who have been riding for 20+ years, but just got deep channel tubeless compatible wheels struggle with flat changes. They're cursing and breaking out the tire lever with 25% of the tire still unseated. I usually just grab the wheel from them, and in 30 seconds I have it on for them with no tools...just a wink and a smile.