How do I keep certain emails from ending up in Junk Mail?

You can 'whitelist' an email address to ensure it is not marked as Junk Mail by Gmail.

In Gmail, if a message is inappropriately identified as spam, you can unmark the message. Just select the message, and click the "Not Spam" button that appears at the top and bottom of your current view. Marking a message as "Not Spam" will result in the email message being automatically moved to your Inbox.

Alternatively, if you would like to prevent this in the future, you can add the sender's email addresses to your Contacts list. Click here for instructions to import your contacts into Gmail.