Rule #1: Be prepared to do your shopping in one day.  Because mortgage rates change daily (and sometime twice daily!), you need to be ready to shop and commit in one day.  All reputable loan officers will be able to issue you a Good Faith Estimate within 30 minutes of quoting you a rate.

Rule #2: If there is no Good Faith Estimate, there is no offer.  Demand a Good Faith Estimate from each loan officer.  A GFE details the costs associated with your mortgage and will help you compare mortgage programs.  Remember that fees and rates go hand in hand.  Typically, lower rates mean higher fees, and higher rates mean lower fees.  Your loan officer will typically make adjustments between rates and fees, if you ask.

Rule #3: Make sure you are comparing identical mortgage products and rates.  A 30-Year Fixed mortgage interest rate cannot be compared to an ARM product interest rate and vice versa.  You would not compare the price of a sit-down dinner to fast-food dinner, even though they are both "dinner".  Same concept.  You have to compare identical products in order to make a fair comparison.

Rule #4: You can have your credit checked an unlimited amount of times within 14 days, so do it.  Your credit scores are only impacted one time for each 14 days when your credit is checked by a registered mortgage lender, regardless of how many times it is pulled.  How much do your scores drop?  Nobody knows for sure, but the scores will not drop until 30 days after the first credit check and that is for your protection.  If a loan officer is telling you not to shop around because your score will drop, you should hang up the phone and never talk to that person again.  If they lie to you before you submit your application, what else will they lie to you about?

Rule #5: Do not float.  The risk is too great.  This is my 10-to-1 Theory.  At the end of a mortgage shopping day, you can choose to lock your rate, or not lock and gamble on a lower rate the following day.  If rates drop by 0.125%, then you win your bet and maybe you save $30.00 per month.  But, if rates go up by 0.125%, you lose your bet and your payment goes up $30.00 per month.  That is an awful feeling, if you've been there.  The 10-to-1 Theory states that the pain of "losing" $30 per month is equal to the joy of "winning" $300 per month.  In short, there are very few good outcomes when you choose not to lock.

Be smart when comparison shopping for mortgages and understand that financial instruments do not operate like digital cameras or mattresses.  Prices change daily and your monthly payment is not set in stone until your rate is locked.