Tips of ESL (non-native) writers

[HordHolm]

That brings up another good point. Be aware of where your native languages conventions differ significantly from English and pay extra attention to that.
Past and present tense is very different in English and Spanish. I work with a few native Spanish speakers and I notice that in their emails on a regular basis.

And perhaps more strangely, be aware of where your native language conventions differ slightly from English, or have areas of significant similarity to English without being the same or damn near. Those small differences can really stick out to a native. For a given value of native, because the other thing to bear in mind is how many different Englishes there are...