I've created a tile entity that (through the consumption of bottles-o-enchanting placed in its inventory) maintains an 'experience level' internal to itself. It seems to be working fine, until the game is closed and reloaded- then the level has been reset to zero.

 

My understanding was that overriding the ReadFromNBT and WriteToNBT methods would allow the level data to be saved, as indeed that seems to be all that is required to save the items:

@Override
	public void writeToNBT(NBTTagCompound nbt) {
	    super.writeToNBT(nbt);

	    NBTTagList list = new NBTTagList();
	    for (int i = 0; i < this.getSizeInventory(); ++i) {
	        if (this.getStackInSlot(i) != null) {
	            NBTTagCompound stackTag = new NBTTagCompound();
	            stackTag.setByte("Slot", (byte) i);
	            this.getStackInSlot(i).writeToNBT(stackTag);
	            list.appendTag(stackTag);
	        }
	    }
	    nbt.setTag("Items", list);
	    
	    NBTTagInt lev = new NBTTagInt(level);
	    nbt.setTag("level", lev);
	}


	@Override
	public void readFromNBT(NBTTagCompound nbt) {
	    super.readFromNBT(nbt);

	    NBTTagList list = nbt.getTagList("Items", 10);
	    for (int i = 0; i < list.tagCount(); ++i) {
	        NBTTagCompound stackTag = list.getCompoundTagAt(i);
	        int slot = stackTag.getByte("Slot") & 255;
	        this.setInventorySlotContents(slot, ItemStack.loadItemStackFromNBT(stackTag));
	    }
	    this.level = nbt.getInteger("level");
	}

Is this really all that is required? More broadly, is any modification or implementation of a network manager required to keep something like this synchronized between clients and servers? There don't seem to be many good tutorials on this subject that don't point to dead source code, etc., and looking at other mods has left me more confused than enlightened by the variety of implementations used.

Alright, I spent hours figuring this one out. There are a myriad of methods you need to implement besides writeToNBT() and readFromNBT(NBTTagCompound). A working example of a luckyblock tileentity is:

 

import net.minecraft.block.state.IBlockState;
import net.minecraft.entity.player.EntityPlayer;
import net.minecraft.nbt.NBTTagCompound;
import net.minecraft.network.NetworkManager;
import net.minecraft.network.play.server.SPacketUpdateTileEntity;
import net.minecraft.tileentity.TileEntity;
import net.minecraft.util.math.BlockPos;
import net.minecraft.world.World;

public class TileEntityLuckyBlock extends TileEntity {
	public static final int MAX_TYPE = 1;
	
	private int luck;
	private int type; // 0: normal, 1: well
	private boolean isPlayerEditing;
	
	public TileEntityLuckyBlock() {
		this.luck = 0;
		this.type = 0;
		this.isPlayerEditing = false;
	}
	
	public int getLuck() {
		return luck;
	}
	
	public void setLuck(int newLuck) {
		luck = newLuck;
		markDirty(); // important
	}
	
	public int getType() {
		return type;
	}
	
	public void setType(int newType) {
		type = newType;
		markDirty(); // important
	}
	
	public boolean isPlayerEditing() {
		return isPlayerEditing;
	}
	
	public void setPlayerEditing(boolean editing) {
		isPlayerEditing = editing; // not marked because this field doesn't need to be persistent
	}
	
	public void executeDrop(World world, BlockPos pos, EntityPlayer breaker) {
		switch(type) {
			case 0:
				LBDrops.executeDrop(luck, world, pos, breaker);
				break;
			case 1:
			{
				EntityPlayer player = world.getClosestPlayer(pos.getX() + 0.5, pos.getY() + 0.5, pos.getZ() + 0.5, 16.0 * 16.0 * 32.0, false);
				if(player != null)
					LBDrops.executeWellDrop(luck, world, pos, player, false);
				break;
			}
		}
	}
	
	/** Everything below here is an absolute MUST have. **/

	@Override
	public boolean shouldRefresh(World world, BlockPos pos, IBlockState oldState, IBlockState newSate) {
		return false;
	}
	
	@Override
	public NBTTagCompound getTileData() {
		return serializeNBT();
	}
	
	@Override
    public SPacketUpdateTileEntity getUpdatePacket() {
        NBTTagCompound nbt = new NBTTagCompound();
        writeToNBT(nbt);
        return new SPacketUpdateTileEntity(this.pos, 1, nbt);
    }
	
	@Override
	public NBTTagCompound getUpdateTag() {
		NBTTagCompound nbt = new NBTTagCompound();
		writeToNBT(nbt);
		return nbt;
	}

    @Override
    public void onDataPacket(NetworkManager manager, SPacketUpdateTileEntity packet) {
        readFromNBT(packet.getNbtCompound());
    }
	
	/** Obviously these mehtods will be different. **/

	@Override
	public NBTTagCompound writeToNBT(NBTTagCompound nbt) {
		super.writeToNBT(nbt);
		nbt.setInteger("luck", luck);
		nbt.setInteger("type", type);
		return nbt;
	}
	
	@Override
	public void readFromNBT(NBTTagCompound nbt) {
		super.readFromNBT(nbt);
		luck = nbt.getInteger("luck");
		type = nbt.getInteger("type");
		isPlayerEditing = false;
	}
}

 

So basically try copying the implementations of shouldRefresh, getTileData, getUpdatePacket, getUpdateTag, and onDataPacket. You will also need to schedule a render update in onDataPacket if you're rendering your tile entity.

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