There's a few ways to handle this but generally looking at how photography works is a good reference point. So the eye doesn't focus on everything all at once and neither does a camera lens. Here are some things that could help, try them out and see what works best for you. A mix will probably give the best result and you can be a lot more subtle with them that way too.
Focus - having the background out of focus will show its not as important as what's in the foreground. Same works in revers.
Contrast - Humans are drawn to high contrast so you want your main point to have the brightest values and darkest values. You can lower the value range in the background. Generally you also want to consider light on dark and dark on light to get the strongest contrast, this is why designing with silhouettes is so strong. This can also work with colour so you could also desaturate your background a tad.
Atmospheric perspective - Over distance more air particles are in between your eyes and the object and you tend to get a slight blue tint. Fog is very commonly used for this as a more extreme example, especially in games as it saves resources on load distance or as for game mechanics.
