Enterprise Car Rental just began a new promotion for its Enterprise Plus members. Between now and February 27, 2015, you can earn double points on qualifying car rentals.
There does not appear to be a limit on the number of points you can earn. However, you do need to be a member of their Enterprise Plus loyalty program. You can sign up here and be automatically registered for this Plus Your Points promo. If you are already a member of Enterprise Plus, you can register for this promotion here.