How do I create a 3rd person camera in XNA?

Because other people are using too complicated code for me to understand, I decided to ask a question from an XNA noob. So, I'm using the spaceship thing that Microsoft use in their tutorials of Going Beyond: XNA Game Studio in 3D, but I decided to create Asteroids (like in lesson 4) without the tutorials. Here's the code:

using System;
using System.Collections.Generic;
using System.Linq;
using Microsoft.Xna.Framework;
using Microsoft.Xna.Framework.Audio;
using Microsoft.Xna.Framework.Content;
using Microsoft.Xna.Framework.GamerServices;
using Microsoft.Xna.Framework.Graphics;
using Microsoft.Xna.Framework.Input;
using Microsoft.Xna.Framework.Media;

namespace WindowsGame2
{
/// <summary>
/// This is the main type for your game
/// </summary>
public class Game1 : Microsoft.Xna.Framework.Game
{
    GraphicsDeviceManager graphics;
    SpriteBatch spriteBatch;

    public Game1()
    {
        graphics = new GraphicsDeviceManager(this);
        Content.RootDirectory = "Content";
    }

    /// <summary>
    /// Allows the game to perform any initialization it needs to before starting to run.
    /// This is where it can query for any required services and load any non-graphic
    /// related content.  Calling base.Initialize will enumerate through any components
    /// and initialize them as well.
    /// </summary>
    protected override void Initialize()
    {
        // TODO: Add your initialization logic here

        base.Initialize();
    }

    /// <summary>
    /// LoadContent will be called once per game and is the place to load
    /// all of your content.
    /// </summary>
    Model myModel;
    SoundEffect soundEngine;
    SoundEffectInstance soundEngineInstance;
    SoundEffect soundHyperspaceActivation;


    float aspectRatio;

    protected override void LoadContent()
    {
        // Create a new SpriteBatch, which can be used to draw textures.
        spriteBatch = new SpriteBatch(GraphicsDevice);

        myModel = Content.Load<Model>("Models\\p1_wedge");
        soundEngine = Content.Load<SoundEffect>("Audio\\Waves\\engine_2");
        soundEngineInstance = soundEngine.CreateInstance();
        soundHyperspaceActivation =
            Content.Load<SoundEffect>("Audio\\Waves\\hyperspace_activate");


        aspectRatio = graphics.GraphicsDevice.Viewport.AspectRatio;
    }

    KeyboardState keyState;



    /// <summary>
    /// UnloadContent will be called once per game and is the place to unload
    /// all content.
    /// </summary>
    protected override void UnloadContent()
    {
        // TODO: Unload any non ContentManager content here
    }

    /// <summary>
    /// Allows the game to run logic such as updating the world,
    /// checking for collisions, gathering input, and playing audio.
    /// </summary>
    /// <param name="gameTime">Provides a snapshot of timing values.</param>

    Vector3 modelVelocity = Vector3.Zero;


    protected override void Update(GameTime gameTime)
    {
        // Allows the game to exit
        if (GamePad.GetState(PlayerIndex.One).Buttons.Back == ButtonState.Pressed)
            this.Exit();

        // Get some input.
        UpdateInput();

        // Add velocity to the current position.
        modelPosition += modelVelocity;

        // Bleed off velocity over time.
        modelVelocity *= 0.95f;



        base.Update(gameTime);
    }

    /// <summary>
    /// This is called when the game should draw itself.
    /// </summary>
    /// <param name="gameTime">Provides a snapshot of timing values.</param>

    Vector3 modelPosition = Vector3.Zero;
    float modelRotation = 0.0f;

    Vector3 cameraPosition = new Vector3(0.0f, 50.0f, 5000.0f);

    protected override void Draw(GameTime gameTime)
    {
        GraphicsDevice.Clear(Color.CornflowerBlue);

        Matrix[] transforms = new Matrix[myModel.Bones.Count];
        myModel.CopyAbsoluteBoneTransformsTo(transforms);

        foreach (ModelMesh mesh in myModel.Meshes)
        {
            foreach (BasicEffect effect in mesh.Effects)
            {
                effect.EnableDefaultLighting();
                effect.World = transforms[mesh.ParentBone.Index] *
                    Matrix.CreateRotationY(modelRotation)
                    * Matrix.CreateTranslation(modelPosition);
                effect.View = Matrix.CreateLookAt(cameraPosition,
                    Vector3.Zero, Vector3.Up);
                effect.Projection = Matrix.CreatePerspectiveFieldOfView(
                    MathHelper.ToRadians(45.0f), aspectRatio,
                    1.0f, 10000.0f);
            }
                mesh.Draw();
        }

        base.Draw(gameTime);
    }

    Vector3 modelVelocityAdd = Vector3.Zero;

    protected void UpdateInput()
    {
        keyState = Keyboard.GetState();
        // Rotate the model using the left arrow button and scale it down
        if (keyState.IsKeyDown(Keys.Left))
            modelRotation += 0.10f;
        else if (keyState.IsKeyDown(Keys.Right))
            modelRotation -= 0.10f;
        else

        // Find out what direction we should be thrusting, 
        // using rotation.
        modelVelocityAdd.X = -(float)Math.Sin(modelRotation);
        modelVelocityAdd.Z = -(float)Math.Cos(modelRotation);

        // Now scale our direction by how hard the trigger is down.
        if (keyState.IsKeyDown(Keys.Up))
            modelVelocityAdd *= 1;
        else


            // Finally, add this vector to our velocity.
            modelVelocity += modelVelocityAdd;

        if (keyState.IsKeyUp(Keys.Up))
        {
            if (soundEngineInstance.State == SoundState.Stopped)
            {
                soundEngineInstance.Volume = 0.75f;
                soundEngineInstance.IsLooped = true;
                soundEngineInstance.Play();
            }
            else
                soundEngineInstance.Resume();
        }
        else if (keyState.IsKeyDown(Keys.Up))
        {
            if (soundEngineInstance.State == SoundState.Playing)
            soundEngineInstance.Pause();

        }
        // In case you get lost, press A to warp back to the center.
        if (keyState.IsKeyDown(Keys.Space))
        {
            modelPosition = Vector3.Zero;
            modelVelocity = Vector3.Zero;
            modelRotation = 0.0f;
            soundHyperspaceActivation.Play();
        }
    }
}

}