To gain more Paragon points during conversations, press L2 when wings appear at the top left of the screen. To gain more Renegade points, press R2 when a star appears.
To easily gain Paragon and Renegade points, unlock training to be able to redistribute your skill points. Talent Mastery will add a specific percentage to your Paragon and Renegade points. Respec and distribute them into your Mastery repeatedly until your Paragon and Renegade are full.