There are a couple ways you can encourage folks to take measures to help you land in Gmail's Inbox. These can be recommended on your "thank you" page where someone is redirected after they subscribe.
- Encourage them to add your "from" email address to their address book.
- Encourage them to look for your message and drag it from Promotions to the Inbox
On a broader note, messages that contain promotional-sounding language are more likely to end up there. Mentioning words like deals, free, discount, limited time, and those sorts of terms, especially in the subject line, mean that your message is more likely to be flagged as promotional.