new MegaJump[MAX_PLAYERS]; //at the top of the script

#define PRESSED(%0) \
(((newkeys & (%0)) == (%0)) && ((oldkeys & (%0)) != (%0)))

public OnPlayerConnect(playerid)
{
    MegaJump[playerid] = 0;
    return 1;
}

public OnPlayerDisconnect(playerid, reason)
{
    MegaJump[playerid] = 0;
    return 1;
}

public OnPlayerKeyStateChange(playerid, newkeys, oldkeys)
{
    if(MegaJump[playerid] == 1)
    {
        if(PRESSED(KEY_JUMP))
        {
            new Float:P[3];
            GetPlayerPos(playerid,P[0],P[1],P[2]);
            GetPlayerVelocity(playerid,P[0],P[1],P[2]);
            SetPlayerVelocity(playerid,P[0],P[1],P[2]+5.0);
        }
    }
    return 1;
}

CMD:megajump(playerid, params[])
{
    if(MegaJump[playerid] == 0)
    {
        //Messages, Sound etc...
        MegaJump[playerid] = 1;
    }
    else if(MegaJump[playerid] == 1)
    {
        //Messages, Sound etc...
        MegaJump[playerid] = 0;
    }
    return 1;
}