A crumble aluminum foil in put into the bowl to give a body to the salad, you can place it on the side or bottom, depend on how you want to present it: either eyes-level, 45 degree or 90 degree. Apart from aluminum foil, you may use a small bowl and put it inside the bowl to make your salad stand up.
For an eyes-level presentation place the aluminum foil by the size, this will let the salad to stand up and hold to that position - this presentation suitable for an eyes-level shot.
A bowl of salad that has been presented from an eyes-level view. To give fresh look to the vegetable, spray a corn syrup with water and use moisturizer spray bottle to give a fresh misty effect. Corn syrup will make the mist stick longer onto the vegetables compared to water. This will give longer fresher look to the vegetables during photography.
Tips: Avoid cutting the lettuce with knife, it will oxidise faster, soak lettuce in icy cold water for a while and tossed it to a fresher look.