I've had the same problem. There's not much you can do about it, because it depends on how gmail is doing its filtering - which you have no control over.

If you're sending email out from an ISP that is on some kind of email black list, you could try switching ISP's. You can try adding extra stuff to your email header, in order to make your message look "less spammy" - but I tried that unsuccessfully. Note that my emails went to other web-based email services fine - it was just gmail that gave me fits.

As an email recipient, you need to check the email and mark it as "not spam" - once that is done, it won't be trashed anymore. Just make sure you alert users (email recipients to be) that they may need to look in their spam folders for that first email...