• Your vehicle must be less than 25 years old on the trade-in date
• Only purchase or lease of new vehicles qualify
• Generally, trade-in vehicles must get 18 or less MPG (some very large pick-up trucks and cargo vans have different requirements)
• Trade-in vehicles must be registered and insured continuously for the full year preceding the trade-in
• You don't need a voucher, dealers will apply a credit at purchase

Check this site frequently for the most recent updates from the government.