Your IPlayerTracker

package your_package;

import java.util.HashMap;

import net.minecraft.entity.player.EntityPlayer;
import net.minecraft.nbt.NBTTagCompound;
import cpw.mods.fml.common.IPlayerTracker;

import path.to.a_class_that_can_store_your_values;

class HelperHelper implements IPlayerTracker{
    public static final instance = new HelperHelper(); //You register this instance in your @Mod-file
                               //by typing
                             //GameRegistry.registerPlayerTracker(HelperHelper.instance);
    private static HashMap<EntityPlayer, a_class_that_can_store_your_values> playerData = new
    HashMap<EntityPlayer, a_class_that_can_store_your_values>(); //This HashMap stores the playerData while the
                                     //player is logged in.

    public static a_class_that_can_store_your_values getValuesFor(EntityPlayer player)
    //With this method you can access the player's data while he is on the server
    {
        return playerData.get(player);
    }

    @Override
    public void onPlayerLogin(EntityPlayer player) //This is the earliest point where you would need the data
    {
        if(player.getEntityData().hasKey("what_so_ever") //Replace what_so_ever with any String you want
        {
            playerData.put(player, readFromNBTStream(player.getEntityData().getCompoundTag("what_so_ever"); //Here the same String as before
        }else{
            playerData.put(player, new a_class_that_can_store_your_values()); //If the player has no
                                              //saved data
                                            //You create new data for him
        }
    }

    @Override
    public void onPlayerLogout(EntityPlayer player) //Here you can save the data for the player
    {
        playerData.remove(player); //You can now "unload" the player as you won't need the data anymore
    }

    private a_class_that_can_store_your_values readFromNBTStream(NBTTagCompound playerCompound) //This returns
                                        //a new instance of your player-data
    {
        a_class_that_can_store_your_values toReturn = new a_class_that_can_store_your_values();
        toReturn.readFromNBTTagCompound(playerCompound);
        return toReturn;
    }


    @Override
    public void onPlayerChangedDimension(EntityPlayer player) {}
    @Override
    public void onPlayerRespawn(EntityPlayer player) {}
}

public class a_class_that_can_store_your_values { //You can put it anywhere you want. Also in other files
    //your values

    public a_class_that_can_store_your_values()
    {
        //set your default values
    }

    public void readFromNBTTagCompound(NBTTagCompound toReadFrom) {
        //read the values you need
    }

    public void writeToNBTTagCompound(NBTTagCompound toWriteTo) {
        //write the values
    }

    //Feature methods to access the player data and all the variables you need
}