The way I currently do it is $x \not | y$, which looks awful. There's got to be something better available.
[Tex/LaTex] What’s a good way to write “x does not divide y”
math-moderelation-symbolssymbols
Related Solutions
Generally, you can often achieve better results by using the \centernot macro from the centernot package instead of the normal \not command.
In this case, however, the stmaryrd package offers the special \longarrownot/\Longarrownot macros you can use for this purpose:
\documentclass{article}
\usepackage{amsmath,amssymb}
\usepackage{centernot}
\usepackage{stmaryrd}
\begin{document}
$\centernot\longrightarrow$ % with centernot
$\longarrownot\longrightarrow$ % with stmaryrd
\end{document}
Result:
centernot: 
, stmaryrd: 
Try this:
$x\nmid y$
It has been known to 'save the day'.
Best Answer
$x\nmid y$saves the day.