The "correct" way is to look at the whole score, to see where you have several measures on a system and one fewer (or one more) won't look bad. Before computers, this was done by hand before any of the final copy of the music was made. The process of splitting up the score into system-sized pieces, and dividing them into pages, was called "casting off". Of course in an orchestral score you might be leaving out staves which are completely empty, so the position of the system breaks might affect the number of staves in each system and how they fit on the pages - in general, deciding on the "best" layout is not a trivial problem to solve.
Ideally, you want the final page to be full of music, not just the final system - and for a long piece, you also want to end on a left-hand page, so you don't "waste" a blank page opposite the inside of the back cover.
Most computer notation software doesn't do any of that automatically. Poor layout is one of the "red flags" that says "this score was made by somebody who either doesn't know how to do it right, or doesn't care".
A professional notation program also allows you to change the relative spacing between different lengths of notes - i.e. the horizontal space between two quarter-notes, compare with the space between two 8ths, two 16ths, etc. That can make big changes to the total amount of space required for the music, without being "obvious" to a casual reader or performer.
As a last resort, you can split up measures so the first part of a measure is at the end of one system and the rest at the start of the next system.
The only situations where all these "tricks" fail are for very short pieces where there just isn't enough music for the changes to have any effect. In that case, a final system that is about half-full of music is more acceptable than s system with one bar that is ridiculously "spaced out" - especially since the last bar of a piece often contains fewer notes than the average bar, and therefore needs less horizontal space.
If you look carefully at professionally engraved scores, you can often see where these techniques have been used. The music doesn't just "fit neatly on the pages" by magic!