[Think]

new oldskin[MAX_PLAYERS];

if(!strcmp(cmdtext, "/sawnoff", true))
{
oldskin[playerid] = GetPlayerSkin(playerid);
SetPlayerSkin(playerid, 55);
return 1;
}

if(!strcmp(cmdtext, "/leave", true))
{
SetPlayerSkin(playerid, oldskin[playerid]);
return 1;
}