Master trail braking. Look it up in the forums, this will give you the biggest gain quickly. All of the fast guys are masters of this.
^ That is your best bet. Nothing can help you more than practice.
Also master tracks, every track has different lines. Some cars may use different lines on the same track. Some tracks have really "odd" lines that looks slow but are actually fast (grand valley is a good example).
If you have a DS3, you most likely need a wheel (DFGT is all you need). Anything more expensive will not make "that" much of a difference.
If you have a wheel with a clutch, go back to using the auto-clutch-blip even if you are a heal-toe master the game will be faster.