Give multiple bribes.
Yep so instead of giving one free report, one video etc, give 3!
And don't send it out all at once. Send one every day or every 2 days.
This prevents people from signing up to your list and instantly unsubscribing just to get the freebie.
Better yet, you can even have a ongoing newsletter as your opt-in bribe. That way they are GIVING you permission to regularly contact them, and they will expect that.