In laying out your pages, avoid creating paragraphs which start on the last line of a page or which finish on the first of the next. These isolated lines of text are called ‘widows and orphans’. The solution to this problem is to set your word processor to control the number of lines on a page so as to push the text forward (turn on the widows and orphans feature). You can also move the paragraph to the next page manually, but do this last thing before you publish the book. It will leave an extra-large gap at the bottom of a page but that will look better than an isolated single line of text.