As you probably know the L consists of 10 images usually presented in two rows of five. I’d start by getting at least 20 if not 30 images to give yourself the best possible selection to choose from. Draft prints at 6×4 on the kitchen table is a great way to start arranging the panel.
The top middle image is usually considered the central image so I would suggest putting your best image there. A good portrait for example works well as it tends to draw the eye. The images around it on both rows should work to lead the eye into the middle of the panel.
A symmetrical arrangement of images is also useful as part of the judging does take the layout of the panel itself into consideration.
Make sure the printing is up to scratch too. The judges will take a close look at the individual shots and poorly printed but otherwise excellent images can cause a panel to fail.
I’d agree with Paul on the printing. It’s amazing how many images look great at a distance but close up are let down by the printing. And the judges do examine them closely. Some even toggle between various sets of glasses they have!
I’ve also mention the mounting. Obviously make sure the mounts are all the same colour but get it done properly. Nothing worse than having a great photo mounted crookedly.