Because most of the techniques have already been mentioned, my tips will generally focus on the car tuning and setups.
Since the Silvia 13 and Silvia 15 are the most used Premium drifting cars, my tips will focus on those cars.
Assuming you already had fun with your car on rainy courses, these are my tips for tuning the S13 and S15 from Cheap to Expensive order..
Obviously, Brakes are the first thing you can tune free of charge:
Unless you are doing high speed drifts, in which case I recommend ABS and gentle weightshift techniques for the less experienced, Myself I dont use ABS no matter what the track layout and condition is.
2 front and 3 rear on your usual silvia is more appropriate, because you wont entirely lock the wheels by accident (Takes a lot of brake training to brake smooth with higher numbers)
2 front and 3 rear is good for braking hard (Just push the brake to the bottom..) While manipulating your steering accordingly.
Or if you need to perform subtle weight shifts without the car going sideways and full brake lock immediately.
But if you want to make sure, why dont you take the car on a rainy course with high speed corners, and see for yourself.
As for Tires, I would recommend Either Comfort Soft or Comfort Medium, Comfort Hard is.. Too hard, and even though they are easier to stay sideways with, the lower grip level means you can easily slide of the track, and, tires that just slides your car all over the place wont teach you anything about proper recovery without any counter movements visible on your replays (counter movements are NOT cool in drifting)
So in other words, higher grip tires teach you a lot about how to properly manipulate steering, brakes, and throttle in more ways than I can ever hope to explain here, you just have to go out and do it.
A good clutch makes Clutch kick, Downshift Rearwheel Locks and other relevant techniques simpler and sharper, and since you also probably need the following throttle response, I also recommend aftermarket Flywheel, as well as a Carbon driveshaft.
For cheap immediate mods you can do ECU, better intake manifold, racing filter, and if you have already come this far, I highly recommend a Limited Slip Differential.
The LS Differential settings depends, if you have over 350 HP, I recommend 25-30 Torque, 60% Shim (Sensivity) for Acceleration and 30 % Shim for Coast (Deacceleration)
For below 350HP, just 60% everything is good, myself I drive with a 60% shimmed LSD all the time no matter how much HP, because I like the car going sideways even if I let off the throttle, thats my style for the highspeed transitions with the Silvia`s, but it could be different for everyone of course.
Assuming you practiced with the car like this for many many hours, and you wish to take the whole tuning concept one step further (If not all the way up) I highly recommend Restoring Body Rigidity at the GT Auto Shop, in order for your car to work better for the new suspension (Which I will get to shortly)
For suspension mods, Just go straight with the sickest adjustable suspension, once it`s on the car, my recommendations for the Suspension settings is the following:
Toe front 0.10, Toe rear 0.05, Camber Front 3.0 to 3.5 (This simply means better grip up front, but not so much that you cant do 4 wheel drift entries at even at 100 kph..), Camber Rear 0.0 to 1.5 (The higher the speeds you will drift at, the higher the rear camber should be, up to 1.5 imho)
As for anti roll bars, my own preference is to have double as much anti rollbars up front compared to the rear, but this is highly user dependent, some likes a stiffer rear and softer front, with higher rear stiffness, you achieve oversteer, whereas high front stiffness, gives more understeer (Means you can push it further when drifting through sleek curves)
As for the springs, I like to leave them at around 10kg front and 8kg rear.
Lower the car as much as the suspension allows, leave extension as it is, but put compression ratios to around 3 on both front and rear (This might depend on individual style, but I like the suspension bumping the rear axle a little because it assures me that crucial power is actually coming to the wheels even if the road conditions are a little rough)
After practicing like this for a while, and you figure you need more power, I recommend simply shedding all excess weight of your Silvia`s body/chassis, add lighter windows, lighter bonnet, go through all 3 stages on weight shaving (With the S13 you can get it down to 965 kg..) As well as add more chassis/body rigidity reinforcement.
If you still figure you want more power after this (Even though the Power to Weight ratio should have improved dramatically)
The first thing you might want after this is a Medium RPM Range Turbo Kit, the High RPM Range Turbo Kit offers no response or kick down below, I don`t even recommend the High RPM kit to advanced users, so as you may figure, with medium range torque you sacrifice a bit of HP and torque high up, but it doesnt matter because you need the extra HP and torque to stay right in the powerband that you regularily use, and since G25/G27 cant upshift during the drifts due to stupid priorities during game development, its a big advantage for the RPM needle not to bump on the rev limiter whilst in the apex of the corner in full four wheel drift... Suit yourself if you want bigger turbo, but you will find that mostly, you cant even use this power so do it at your own risk and don`t come and whine to me whether you can or cant drift with it, I take distance from it!
After these mods and youre still hungry for power, comes simply modding the engine through all 3 stages..
Thats all I can think of for the moment being
(true story).
