Packets are Easy (by Dany)

ModClass.java

@EventHandler
public static void init(FMLInitializationEvent e)
{
  // this should be in initialization
  PacketHandler.initPackets();
}

PacketHandler.java

public class PacketHandler
{
  public static SimpleNetworkWrapper net;
  
  public static void initPackets()
  {
    net = NetworkRegistry.INSTANCE.newSimpleChannel("YourModId".toUpperCase());
    registerMessage(SimplePacket.class, SimplePacket.SimpleMessage.class);
  }
  
  private static int nextPacketId = 0;
  
  private static void registerMessage(Class packet, Class message)
  {
    net.registerMessage(packet, message, nextPacketId, Side.CLIENT);
    net.registerMessage(packet, message, nextPacketId, Side.SERVER);
    nextPacketId++;
  }
}

SimplePacket.java

public class SimplePacket implements IMessageHandler<SimpleMessage, IMessage>
{
  @Override
	public IMessage onMessage(SimpleMessage message, MessageContext ctx)
	{
	  // just to make sure that the side is correct
	  if (ctx.side.isClient())
	  {
	    int integer = message.simpleInt;
	    boolean bool = message.simpleBool;
	  }
	}
  
  public static class SimpleMessage implements IMessage
  {
    private int simpleInt;
    private boolean simpleBool;
    
    // this constructor is required otherwise you'll get errors (used somewhere in fml through reflection)
    public SimpleMessage() {}
    
    public SimpleMessage(int simpleInt, boolean simpleBool)
    {
      this.simpleInt = simpleInt;
      this.simpleBool = simpleBool;
    }
    
    @Override
    public void fromBytes(ByteBuf buf)
    {
      // the order is important
      this.simpleInt = buf.readInt();
      this.simpleBool = buf.readBoolean();
    }
    
    @Override
    public void toBytes(ByteBuf buf)
    {
      buf.writeInt(simpleInt);
      buf.writeBoolean(simpleBool);
    }
  }
}

UseExample.java

// Sending packet to server
IMessage msg = new SimplePacket.SimpleMessage(500, true);
PacketHandler.net.sendToServer(msg);

// Sending packet to client
if (player instanceof EntityPlayerMP)
{
  IMessage msg = new SimplePacket.SimpleMessage(800, false);
  PacketHandler.net.sendTo(msg, (EntityPlayerMP)player);
}