\relpenalty = 500 \binoppenalty = 700
You make the break progressively less attractive by increasing these values. You can actually forbid all breaks, everywhere, by:
\relpenalty = 10000 \binoppenalty = 10000
If you want just to prevent breaks in a single expression, write:
{% \relpenalty = 10000 \binoppenalty = 10000 $a=b+c$ }
and the original values will remain undisturbed outside the braces. This is tedious: there is often value in an alternative approach, in which you say which parts of the expression may not break whatever happens, and fortunately this is surprisingly easy. Suppose we want to defer a break until after the equality, we could write:
${a+b+c+d} = z+y+x+w$
The braces say “treat this subformula as one atom” and (in TeX at
least) atoms don’t get split: not a \
binoppenalty
change in sight.
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=brkinline