Use mocks sparingly and keep them simple! Never make a mock more complex. From http://hjerpbakk.com/blog/2013/6/21/ndc-2013-day-3.html

GoodAndBadMocks.cs

[TestClass]
public class MockExample
{
    /// <summary>
    /// An interface for an imaginary stock service.
    /// </summary>
    public interface IStockService
    {
        double QuoteForTicker(string ticker);
    }

    /// <summary>
    /// Class representing an application model.
    /// Utilizes the IStockService in its implementation.
    /// </summary>
    public class SampleApplicationModel
    {
        private readonly IStockService stockService;

        public SampleApplicationModel(IStockService stockService)
        {
            this.stockService = stockService;
        }

        public string CurrentTicker { get; set; }
        public string LatestQuote { get; set; }

        public void GetQuote()
        {
            var quote = stockService.QuoteForTicker(CurrentTicker);
            LatestQuote = quote.ToString();
        }
    }
   
    [TestMethod]
    public void GetQuoteMustSetLatestQuoteForTicker()
    {
        // Simple mock, using an appropriate tool
        var stockServiceFake = new Mock<IStockService>();
        stockServiceFake.Setup(s => s.QuoteForTicker("AAPL")).Returns(700D);
        var model = new SampleApplicationModel(stockServiceFake.Object) { CurrentTicker = "AAPL" };

        // Mock with internal logic
        //var model = new SampleApplicationModel(new StockServiceFake()) { CurrentTicker = "AAPL" };

        model.GetQuote();

        Assert.AreEqual("700", model.LatestQuote);
    }

    [TestMethod]
    [ExpectedException(typeof(InvalidOperationException))]
    public void GetQuoteLFailsMustSetErrorMessageAsQuoteValue()
    {
        // Simple mock, using an appropriate tool
        var stockServiceFake = new Mock<IStockService>();
        stockServiceFake.Setup(s => s.QuoteForTicker("AAPL")).Throws(new InvalidOperationException());
        var model = new SampleApplicationModel(stockServiceFake.Object) { CurrentTicker = "AAPL" };

        // Mock with internal logic
        //var model = new SampleApplicationModel(new StockServiceFake(true)) { CurrentTicker = "AAPL" };

        model.GetQuote();
    }

    /// <summary>
    /// Complicated mock. Hard to understand, must be maintained
    /// together with the interface it's implementing.
    /// </summary>
    private class StockServiceFake : IStockService
    {
        private readonly bool throwException;

        public StockServiceFake(bool throwException = false)
        {
            this.throwException = throwException;
        }

        public double QuoteForTicker(string ticker)
        {
            if (throwException) 
            {
                throw new InvalidOperationException();
            }

            return 700D;
        }
    }
}