Si no chingas, te chingan

This is another classic Mexican expression based on the verb chingar.  Big surprise huh?

Let's get straight to the point (ir al grano) and start translating our expression.

Let's look at a similar English expression:

Do onto your neighbor before he does onto you

I'd say that translation truly captures the essence of our expression, with one difference - it's not entirely 100% accurate because it's not vulgar enough.  Remember that anything involving chingar is very, very rude.

In this context chingar is pretty much directly translates to fuck in English.   In fact, the English expression we're looking for here is "to fuck someone over", or simply "to fuck someone" (to do something bad to them).

With that in mind, let's make one more try at translation:

Si no chingas, te chingan
If you don't fuck them, they'll fuck you.

That's more like it. 

As far rude expressions go, this one was pretty straight-forward. 

¡Hasta pronto, y espera que te sirva!