One of the biggest problems I have when assembling deep cabinets is centring a long clamp on a somewhat small surface, like the edge of plywood. After going back and forth several times to each end of the clamp to keep it centred, I thought there had to be a better way.
The solution I came up with uses another clamp as a third hand. I use a small clamp to centre and capture one end of the clamp, as in the photo above. This allows me to accurately position and tighten the clamp from the opposite end. Then I can remove the small clamp and move on to the next one.