H E R E W E G O
(1) ok so if you have a good piece of art (albeit small) that you've somehow put SMACK DAB in the middle of a paper: try using sticky-note doodles around it or draw a frame around it and continue to use the rest of the paper!
(2) comics take a lot of space. try to format them to take a lot of space! (if you need help with that it needs a whole different answer)
(3) i know val (@queenvalerie_) cuts out notebook doodles and puts them in her sketchbook in case you want to do that :)
(4) if you've done most of your art in one medium, try really small thumbnails or like "trial color swatches" in your free spaces !
(5) remember: your sketchbook is your own. and it's unique! however you decide to fill it is ok, as long as you are content with it
xoxox