"There's a lot of excitement and emotion around an engagement, and you can get sucked into the whirlwind," said Alan Fields, co-author of "Bridal Bargains."
Before you start visiting venues or shopping for a dress, settle out some money issues—including, who's paying. More couples are financing the bulk of the celebration themselves these days, with family making some contributions. It's smart to determine how much money you're working with before committing to a pricey purchase that could eat up a big portion of the budget. (See chart above for some of the biggest budget line items.)
Then prioritize. Your budget might not allow for top-of-the-line everything, but if having say, great photos is a must, reallocate the budget to spend less on invitations and favors and more to hire a better photographer. Many vendors are willing to work within your budget, so ask what your options are if their rates are just a little out of reach.
Read More7 of the priciest weddings—for guests
But there's another number you need to know before starting the full-force planning. How many guests will you have? Even a few unexpected extras can generate hundreds of dollars in extra costs. So make that list early, and have those tough conversations with your significant other about whether second cousins merit an invite, how many family friends your parents can invite and who merits a plus-one.
"A lot of the cost is driven by how many people you have," Fields said. "That's a common mistake that people make."
—By CNBC's Kelli B. Grant.