Short answer: make it thicker.
Long answer: The moment of inertia affects the beam's ability to resist flexing.
Use one of the many, free, online moment of inertia calculators (like this one) to see how increasing the height of the beam will have an exponential effect on increasing the stiffness of the beam.
And this site helps provide a pictorial view of the load(s) upon a beam depending upon differing configurations, such as where the supports are and where the load is applied. It also provides a calculator to determine the forces involved.
Wikipedia has a decent article for area moments of inertia. In your particular case, you're asking about a
filled rectangular area and Ix = bh3/12. The height has an exponential factor of 3 whereas increasing the base does not have an exponential factor. So for the same amount of material, increasing the height stiffens the beam better.
To be clear, you can make the beam sag less by increasing the width of the plate. It's just more effective to make the plate thicker.
Current moment: Ix = 30 * 23 / 12 = 20 mm4
Increase width by 1mm: Ix = 31 * 23 / 12 = 20.6 mm4
Increase height by 1mm: Ix = 30 * 33 / 12 = 67.5 mm4
And if for some reason you can't easily increase the thickness of the plate, you can consider a different beam structure. Currently, your beam is a simple rectangle. You can easily use a T-beam or an I-beam in order to stiffen the plate instead.
Again, while I've provided some suggested links to online calculators feel free to search for and use others that you may prefer.