There are many ways to 'push' the loomed rows together, keeping them straight and even. The two points needing to keep in mind is:
#1 Not too tight, or your looming will have 'stripes' when you cut it off the loom.
#2 Not too loose, or your edges will not be even and straight.
The best way I have found to 'snug the rows of looming together perfectly' is to use my needle.
Before your needle totally exits the row, keep it in place and push the row up snug, then continue to pull the needle through. This will give you leverage because the needle is inside the beads, but sitting on 'top' of the warps. Once the needle is withdrawn, the beads will rest down, in between the rows, and harder to push up properly.
You will also want tug on the weft, after the weft is completely run through the row, to snug the weft properly. I realize how it may work with textile looming, leaving a loose loop on the outside edges, but with bead looming, it is important to pull the weft taught on every pass.
I have been using this same picture of looming, to explain many techniques. I promise to offer some 'meat with the potatoes', as I complete this cuff and share the finished looming!