How can I keep Gmail from viewing legitimate mail as spam?

I’ve recently seen a large increase in my daily e-mail. Despite telling Gmail the sender is okay to send me e-mail, every day I have to retrieve a few messages from the Spam folder. This is particularly frustrating as I forward this e-mail to my main address and some is getting stuck in the first spam filter and some is getting stuck in the second.

Aside from listing the sender as someone I’m expecting e-mail from, is there anything I can do to stop this? Do you know if Gmail will just learn after I keep “not spam“ing them that it is legitimate mail?