It's called adding "Air".

Literally all you do is put an EQ on either your master channel (quicker), or on any applicable instruments (arguably better), and add a slight boost in the high frequency range. I suggest anywhere from 8000 to 20000 Hz, whatever sounds best. This trick will work for many (probably even the majority of) songs, but it isn't always necessary. In my experience, this is usually better on instruments than white noise, but you can usually just slap this onto your master track and be just fine.

This will make your song sound much neater and brighter, and is actually very common with professionals!

Have fun brightening up your songs! :D