fly.inc

An example:
#include <a_samp>
#include <fly>

public OnFilterScriptInit()
{
    print("=========================================\n");
    print("Fly include demo FS by Norck");
    print(" ");
    print("=========================================\n");
    return 1;
}
public OnPlayerConnect(playerid)
{
    InitFly(playerid);
    return 1;
}
public OnPlayerCommandText(playerid, cmdtext[])
{
    if(!strcmp(cmdtext,"/fly",true))
    {
        StartFly(playerid);
        return 1;
    }
    if(!strcmp(cmdtext,"/stopfly",true))
    {
        StopFly(playerid);
        return 1;
    }
    return 0;
}

And I include everything:
// =======================================================================================
// fly.inc
//
// Author: Norck
//
// msn:     paul_norck@hotmail.com
// icq:     44-055-21
// skype:   paul_norck
//
// Credits: Creator of SetPlayerLookAt function. Sorry, I can't remember their name
//
// you are allowed to edit this script
// you are not allowed to sell this script
//
// Please, don't remove the credits
// =======================================================================================

// variables

static bool:OnFly[MAX_PLAYERS];     // true = player is flying, false = player on foot

// prototypes

forward InitFly(playerid);                          // call this function in OnPlayerConnect
forward bool:StartFly(playerid);                    // start flying
forward Fly(playerid);                              // timer
forward bool:StopFly(playerid);                     // stop flying
forward static SetPlayerLookAt(playerid,Float:x,Float:y);   // set player face position

// functions

InitFly(playerid)
{
    OnFly[playerid] = false;
    return;
}

bool:StartFly(playerid)
{
    if(OnFly[playerid])
        return false;
    OnFly[playerid] = true;
    new Float:x,Float:y,Float:z;
    GetPlayerPos(playerid,x,y,z);
    SetPlayerPos(playerid,x,y,z+5.0);
    ApplyAnimation(playerid,"PARACHUTE","PARA_steerR",6.1,1,1,1,1,0,1);
    Fly(playerid);
    GameTextForPlayer(playerid,"~y~Fly mode~n~~r~~k~~PED_FIREWEAPON~ ~w~- increase height~n~~r~RMB ~w~- reduce height~n~\
    ~r~~k~~PED_SPRINT~ ~w~- increase speed~n~\
    ~r~~k~~SNEAK_ABOUT~ ~w~- reduce speed",10000,3);
    return true;
}

public Fly(playerid)
{
    if(!IsPlayerConnected(playerid))
        return 1;
    new k, ud,lr;
    GetPlayerKeys(playerid,k,ud,lr);
    new Float:v_x,Float:v_y,Float:v_z,
        Float:x,Float:y,Float:z;
    if(ud < 0)  // forward
    {
        GetPlayerCameraFrontVector(playerid,x,y,z);
        v_x = x+0.1;
        v_y = y+0.1;
    }
    if(k & 128) // down
        v_z = -0.2;
    else if(k & KEY_FIRE)   // up
        v_z = 0.2;
    if(k & KEY_WALK)    // slow
    {
        v_x /=5.0;
        v_y /=5.0;
        v_z /=5.0;
    }
    if(k & KEY_SPRINT)  // fast
    {
        v_x *=4.0;
        v_y *=4.0;
        v_z *=4.0;
    }
    if(v_z == 0.0)
        v_z = 0.025;
    SetPlayerVelocity(playerid,v_x,v_y,v_z);
    if(v_x == 0 && v_y == 0)
    {
        if(GetPlayerAnimationIndex(playerid) == 959)
            ApplyAnimation(playerid,"PARACHUTE","PARA_steerR",6.1,1,1,1,1,0,1);
    }
    else
    {
        GetPlayerCameraFrontVector(playerid,v_x,v_y,v_z);
        GetPlayerCameraPos(playerid,x,y,z);
        SetPlayerLookAt(playerid,v_x*500.0+x,v_y*500.0+y);
        if(GetPlayerAnimationIndex(playerid) != 959)
            ApplyAnimation(playerid,"PARACHUTE","FALL_SkyDive_Accel",6.1,1,1,1,1,0,1);
    }
    if(OnFly[playerid])
        SetTimerEx("Fly",100,false,"i",playerid);
    return 1;
}

bool:StopFly(playerid)
{
    if(!OnFly[playerid])
        return false;
    new Float:x,Float:y,Float:z;
    GetPlayerPos(playerid,x,y,z);
    SetPlayerPos(playerid,x,y,z);
    OnFly[playerid] = false;
    return true;
}

static SetPlayerLookAt(playerid,Float:x,Float:y)
{
    new Float:Px, Float:Py, Float: Pa;
    GetPlayerPos(playerid, Px, Py, Pa);
    Pa = floatabs(atan((y-Py)/(x-Px)));
    if (x <= Px && y >= Py)         Pa = floatsub(180.0, Pa);
    else if (x < Px && y < Py)      Pa = floatadd(Pa, 180.0);
    else if (x >= Px && y <= Py)    Pa = floatsub(360.0, Pa);
    Pa = floatsub(Pa, 90.0);
    if (Pa >= 360.0)
        Pa = floatsub(Pa, 360.0);
    SetPlayerFacingAngle(playerid, Pa);
    return;
}