Here are a couple from me:
1. Make clever use of perk points to get discount on cars or parts, level up more quickly or improve performance. You can redeem the points and re-use them.
2. When you reach a certain level in the game (10, 20 etc), you can buy a complete level kit for all your cars, so you do not need to level up every car individually.
3. Unlocking radar stations also unlocks the hiding spots for Kit Cars. Use the filters to discover more hidden secrets on the map (thx Luny)