Adding depth to your banner designs will help them to appear more professional and will guarantee that they're a step above the rest. When we think of depth, the first thing that comes to mind is perspective in illustration, but you can also achieve this by paying attention to the way all of your elements fit together.
Adding texture to your banners is one way to make sure the background is more interesting and, at the same time, it helps your main elements to stand out against something other than a plain flat color. You can search for or create your own high resolution textures. When searching for textures, you should make sure that, if they're raster images, they're high enough resolution to fit the size banner that you're creating. Otherwise, the background will look pixelated.
Seamless patterns are another way to guarantee your texture spans across the entire size of the banner. Seamless patterns are swatches created in a way that all sides meet up to create a repeating pattern that tiles itself throughout a shape in the same way a color fill would.
Whichever you use, it's always best to keep textures subtle. Make sure it falls into a muted color and doesn't take away from the main elements of the banner.
A simple design can be made to look professional and clean when using gradients. In the same way that textures should be made subtle, a gradient is also best when used sparingly. Subtle gradients are modern, especially when used with a pleasing color palette. Let the colors blend together slowly and over a large span of space.
Try using images in the background in a way where they drop out of focus. You can blur the image and put it behind the main elements or pull back the transparency to make the object drop into the background.
Another thing to remember is that different colors will cary different visual waits. This means that things like cool, dark colors tend to recede into the background of the design, while warm, bright colors tend to move to the forefront.