Throughout your life, there will be times when you lay down some serious cash all at once. It is at these critical junctures when your deodorant -- strong enough for a mule but made for a human -- will truly prove its mettle.

If you don't plan ahead and just hope you'll have the money when the purchase is upon you, be ready for disappointment, checking-account balance shock ... or serious debt.

No one knows better than I do how an innocent trip to Target (NYSE:TGT) can turn into a $237.49 bill. Obviously, every little purchase can't be planned. But if you scope out the big ones, you can parlay three willy-nilly Target trips into a flat-screen TV with little effort. Here are some commonsense ways to prepare to shell out the big bucks.

Commit your dreams to paper
Think about what pricey items or services might be on your horizon. You don't need a crystal ball, though you'd be a hit at the track. Simply brainstorm with your significant other or on your own. (Keep your cards in your wallet -- this is a virtual trip to the mall.)

After you have a master shopping list, divide it into three categories: "Must-haves," "Nice-to-haves," "If I win the Lotto." Next to each purchase put a realistic price range -- the more accurate the better, though, there's no need to exhaustively comparison-shop at this time. For appliances, cars, couches, and home electronics, simply flip through that pound of fliers from the Sunday paper to get a sense of what stuff really costs.

Once you have your list, set some target acquisition dates. These can be three weeks, three months, or three years from now. I suggest you write this part in pencil. You'll see why in a moment.

Break it down
When you know how much you need and when you need it, break the purchase price down into monthly savings goals. Simply divide the item's cost by the number of months you have until purchase.

Inhale. Exhale. Inhale. Exhale. Deep, cleansing breath.

After you recover from sticker shock, tweak those dates. Here's where you really have the chance to see what it takes to live within your means. Sure, your neighbors may have the latest electronic gizmos and a cutting-edge car that runs on lilac-scented bath water. But you don't know the state of their finances -- their debt, the number of mortgages they carry, how much they've borrowed from their brother-in-law. But you'll probably have a pretty good idea after this exercise.

Park your cash
Once you've come up with a doable amount to set aside each month for your planned big purchases, find an appropriate place for your savings.

"In the checking account" is probably a bad idea. (See "Target pit stop" discussion above.) And while it's tempting to try to get a big bang for your bucks in the stock market, that's not the right place for this short-term money, either. Over the short term, the market is too volatile for short-term savings. Would you have wanted to have your emergency fund, or your house down payment, or your kid's tuition in the Nasdaq at its height on March 10, 2000? If you can't afford or can't wait out a 30% drop in the value of your savings, keep it out of the market.

So where should this money go? Answer: In the most boring place possible.

There are a wide variety of instruments for your short-term savings, including money market accounts, certificates of deposit (CDs), government and corporate bonds, and bond mutual funds. We cover these different choices in greater depth in Where to Stash Your Cash.

There's no better way to build up your big-purchase fund than to automate it. If you're like me, out of checking account means out of mind. When you shop for your short-term savings account, see if the institution can accommodate automatic balance transfers. You pick the day and amount; the magical bank wizards do the rest.

Consider other options
What happens when your wish list is -- how shall I put this politely? -- unrealistic? Here are your options:

  1. Scale down your goal. Perhaps you should aim for a rowboat instead of a motorboat.

  2. Move back your target date. Is the goal more attainable if you put off the purchase and save for another year or two?

  3. Spend time looking for cheaper options. While you're socking money away, spend the time investigating ways to save money on your purchase.

  4. Don't do it. Maybe in the end it's not worth the time, effort, and financial sacrifice.

Even if you scrap a savings goal, this exercise in planning will leave you better off than if you simply plunked down the big bucks with little forethought.

For in-depth guidance on all sorts of money conundrums, see The Motley Fool's Guide to Personal Finance .

Dayana Yochim is the author of Couples & Cash: How to Handle Money With Your Honey . She owns none of the items mentioned in this article, but is saving for a really cute pair of summer shoes. The Motley Fool is savers writing for other savers.