The Nine Most Important Rules when Applying for a Mortgage

Do not change jobs, become self-employed or quit your job.

Do not buy a car, truck or van.

Do not use charge cards excessively or let your accounts fall behind.

Do not spend money set aside for closing.

Do not stop making your mortgage payments while waiting for a refinance to go through.

Do not agree to have your credit be run.

Do not make large deposits, cash or otherwise, into your savings or checking account unless it is from your paycheck(s) or you can verify their source. 

Do not change bank accounts

Do not co-sign for a loan for anyone.