Keep a file of inspiring work.
This can be photos of superbly-executed road-going or concept vehicles, highly-engineered race vehicles, construction machinery, architecture, fashion, electronics, graphic design, photography, paintings, sculptures, sketches or other transportation portfolios.
Always be on the lookout to add more reference to your own catalogue. Get a varied cross-section of the existing sketch and render styles prevalent in the industry so you don’t develop something too derivative or contrived.
For hard copy reference, books and magazines are hard to beat.
Here are some ultra rad periodicals: Car, EVO, TopGear, Car Styling
Here are some excllent books: Car Design Yearbooks, H-Point, How to Design Cars Like a Pro
Refer to this collection whenever you find yourself stumbling over how a reflection might appear or how to resolve a bevel or tricky surface transition.