Split/Second

A few general remarks from me regarding perfecting all races, adding on to Ramon's tips.
For me, the last ones I had to complete were Season 4's Detonator (Power Plant), Season 12's 2nd race (Dam), Air Revenge (Graveyard) and Elite Race. These were definitely the hardest for me, so I'll try to give some specific tips too.

Races/Eliminators:
Generally easy, just a few are tricky. The main hint I can give is: TAKE OUT THE FIRST CAR(S) ASAP, before they get too much ahead of you and you can't use powerplays on them anymore. Do this in the first 30 or so seconds of a race. The main reason for not winning a race is mostly that you're just not able to catch up with the first dudes, don't let that happen and put them down fast.
ALWAYS USE SHORTCUTS. you can't afford to miss one. If you don't do it, the guy behind you will. On the other hand, don't risk trying to go under one that is about to close, it almost never works.
NEVER USE RED POWERPLAYS, they're not worth their cost in power, ONLY ROUTECHANGERS are worth it and can work great to turn the tables. Don't forgo a shortcut or the opportunity to take out the pole position cars just so you can use a route changer, remember that everybody will be able to use the altered route.
Some tips for the hardest tracks:
-PowerPlant: most dangerous area is the warehouses that completely collapse. It's frequently worth it to drop back a few positions and bomb your opponents with this one, to prevent them from using it on you. The short cut is the most important thing in the race though, it can completely save your ass.
-Downtown Central
The extreme series of 90 degree corners at the end is very tricky, you need to be able to take these really well, which means starting your drift in advance (timing requires practice).
That said, key strategy here is to be in last place at the end of the first lap, with full power. Then just drop the tower on everybody, and race as perfectly as you can. I've never been able to beat this race (2 laps) on higher difficulties without doing this.
-Canyon/Dam
Very hard because of the cliffsides you can drop off and since more than 75% of powerplays is almost impossible to avoid and leave debris.
Take out the first guy as soon as you can, this has never mattered more than in the S12 Canyon race. If you're able to take out the majority of the cars in the first stretch, you might be able to stay ahead permanently. If you're ahead by more than 4 seconds, you're in the clear.
Unfortunately this race might just be a fair bit of luck, it took me more than 50 tries to come out as 1st... (I restart when I end up last halfway though)

Detonators:
Ramon makes good points: NO DRIFTING! Unless there is no other way or the car just sucks too mush (cough ryback coyote). Also don't brake or slow down excessively for corners, this can easily end up even slower than drifting.
You will need extreme attention to detail here, it's really repetition untill you know it in and out. Sometimes there are tricks that can save just the necessary 5/10ths of a second.
Try to keep track of "checkpoints" halfway to judge your performance: don't complete a detonator if you know you're 2 seconds behind on you best time.
For the S4 PowerPlant detonator (took me well over 70 tries), some specifics:
-Turn lightly, almost tapping if you can, it prevents the car from drifting.
-At the first 90deg corner (the first serious one), into the warehouses, it really pays off to cut this corner as much as you can, instead of sliding it in advance. It saved me almost a second.
-Sliding under the dropping girder-stack is very tricky, you could blow up if you slide in too hard, timing is crucial. Also, aim to drift under the exact center.
-The corner just before the cooling towers is almost impossible to do right, just don't turn in too soon, it's better to hit the outside than the inside.
-When drifting around the first cooling tower, try to countersteer so you don't hit the wall just before you go into the second tower's turn.
-Just before the shortcut, learn to cut the corner there, and try to countersteer enough so you can take the next corner.
-Last turn is very important, you really have to learn to time this one in advance. While you can actually take this one while barely hitting the wall (I pulled it off once and have seen others do it), it's not required for the 1.21 time.

Air Strikes are easy, never had trouble and you shouldn't either

Air Revenge is another story. The last one in S12 (Airplane Graveyard) gets very hard. The right car can make a big difference here, which would be the COBRETTI SLIPSTREAM. To make the difference in time, you need to drift for extra power on top of the Perfect Wave bonus. The Slipstream is one of the best "auto-drift" cars and while it's slower than the 530GTS, it's easier to control.
Another key trick is to WAIT FOR THE HELI TO GET AHEAD AT THE START. Time doesn't start ticking until you cross the start line, that way, if you get out of the hangar, you can start dodging missiles sooner, shaving off a few seconds. This only works if the start is in a covered area, otherwise the Heli just blows you up ;)
With that said, just try to get a perfect wave every time, especially the first 2 or three waves. You can win while missing a few though.
And yeah, always go for the red strike, it's worth your power more.

Survival isn't that hard either, just requires some skill in judging the best passage. Staying away from the truck as far as possible when passing always helps.
Best car for survival IMO is the Hanzo FX350, as it never drifts unwantedly, while turning very sharp. Best car to use for the 2 Survival achievements also.

That's about it, my tips for getting all gold. It's tough, but doable, up there with Burnout Revenge and FlatOut UC's "All Gold" achievements ;)

For this achievement you have to be first in every single event in the season mode for all 12 episodes.

(Elite)Races:
Pretty much racing is all you have to do. Keep in mind to have fun and especially with the last couple of races it takes a couple of tries. Also don't worry if you can't beat the first races directly because you get better cars down the road.

Eliminator:
In eliminator I found it very easy when you save up your power to use at the first big red powerplay that comes. In the beginning the pack is closer together and with a big powerplay you can take them all out at once and stay ahead. As far as the rubberbanding goes. When you're first, save your power for a rival that passes you. Never switch routes when your first and try to use as much shortcuts as you can.

Air Strike:
Important to know that you don't have to be too far away from the red target areas. In fact you can drive past them with only inches to spare without being hit by a shockwave. As long as you don't hit the markers you'll do fine. Slowing down a little in the turns helps out too.

Air Revenge:
Same as the above but never use the level 1 powerplays as they always remove a single bar from the helicopters health whilst the level 2 powerplay removes 4. Also in the last air revenge drift alot!

Survival:
This is a case of calming down, just relax and concentrate. In the beginning you can get away with staying on the far right or far left of the track but after a couple of trucks you're better off near the middle, driving right in between the barrels. Also remember when the truck is driving left, you should be right and vice versa. It's ok to drive slower or stay behind the next truck to wait for the barrels to stop rolling.

Detonator:
After a couple of tries you’ll get to know the track and its powerplays. The only advice i can give you is never drift because drifting slows you down and its actually only useful when you need power. When you approach a turn just release the gas and make the turn passing the left side of the turn and ending up on the right or vice versa. As soon as you're car lines up straight, floor it. It's also possible to release the gas slightly instead of letting it completely go which is useful for the wider and bigger turns. Try it a couple of times and it'll be a piece of cake.

Hope this helps! And don't forget to have fun!

Xoliul is right on everything, but I have an adjustment to make to his detonator strategy. Some cars are faster without drifting, but cars such as the Hanzo FX350, Cobretti Cascade , and especially the Ryback Firestorm need to drift to go faster. For example, in the canyon detonator, I have a 1:15 in a Firestorm drifting but I have a much slower record not drifting.