[Tr1viUm]

These function can also be used for players. Just change the functions to the ones that belong to players. GetVehicleVelocity will be GetPlayerVelocity... so on.

BoostVehicle
You can increase/decrease vehicle speed with this function. It's recommended to not go higher than 2.0 or lower than -2.0. Using 0.25 as speed is fine.

Code:

stock BoostVehicle(vehicleid, Float:speed){
	new Float:X, Float:Y, Float:Z, Float:A;
	GetVehicleZAngle(vehicleid, A);
	GetVehicleVelocity(vehicleid, X, Y, Z);
	X += (speed * floatsin(-A, degrees));
	Y += (speed * floatcos(-A, degrees));
	SetVehicleVelocity(vehicleid, X, Y, Z);
}

VehicleJump
You can make a vehicle go up or down. I usually set height to 1.0.

Code:

stock VehicleJump(vehicleid, Float:height){
	new Float:X, Float:Y, Float:Z;
	GetVehicleVelocity(vehicleid, X, Y, Z);
	SetVehicleVelocity(vehicleid, X, Y, Z+height);
}

StopVehicle
This surprisingly stops the vehicle... :P

Code:

stock StopVehicle(vehicleid){
	SetVehicleVelocity(vehicleid, 0.0, 0.0, 0.0);
}