4. Bring some cash to the table

The 20% down payment is once again king. Put down less than that, and you'll have to buy private mortgage insurance, which will increase your monthly payments.

Many people with less than 20% are being funneled into FHA loans, which come with higher rates than conventional loans but offer you the option of putting as little as 3.5% and saving any extra accumulated cash.

Talk through the possibilities with your loan officer, as a smaller down payment could result in a higher rate but more overall financial flexibility, since you'd have more cash left over after closing to cover emergencies, maintenance and repairs.

I believe a 20% down payment is the smartest way to go, if you can swing it. But I also think it's wise to have a good pile of cash left over after you buy a home, because you're going to need it.

5. Get real about the house's value

In Econ 101, you might have learned that fair market value is the price a willing and knowledgeable buyer would pay a willing and knowledgeable seller when neither is under compulsion to buy or sell.

In today's mortgage world, however, the value of a house is what the appraiser says it is, and I'm hearing a lot of complaining that appraisers have gotten pretty conservative lately.

Why does this matter? If the appraiser says a house is worth less than what you agreed to pay, you'll have to cough up more money for the down payment to make up the difference or go back to the negotiating table with the seller (as if it weren't torture enough the first time around).

Protect yourself as much as possible by thoroughly researching sales of comparable properties before you make your bid. Your real-estate agent should be able to help.

There are other ways your mortgage deal can go south, even if you try to get your financial and credit situations in the best possible shape.

Liz Weston is the Web's most-read personal-finance writer. She is the author of several books, most recently "The 10 Commandments of Money: Survive and Thrive in the New Economy" (find it on Bing). Weston's award-winning columns appear every Monday and Thursday, exclusively on MSN Money. Join the conversation and send in your financial questions on Liz Weston's Facebook fan page.