LONDON-- Time and time again, when deciding whether to invest in a stock, I find myself thinking "if only it were a little bit cheaper." I thought it just the other day when writing about Unilever (LSE: ULVR.L).

This household-goods giant is cleaning up in emerging markets, which can't get enough of brands such as Knorr, Lipton's, Ben & Jerry's, Persil, and Dove soap. Better still, it yields 4%. There's only one thing wrong: It costs too much. With shares trading on a price-to-earnings ratio of 18, I fear the potential for further capital growth is limited.

Dash for crash
Too many stocks I've been sizing up just look that little bit too expensive. That's largely down to the summer rally, which saw the FTSE 100 leap more than 12% to around 5,900, with some stocks doing far better still. Barclays, for example, leapt 50%.

I hate paying over the odds for a stock or getting suckered by a short-lived rally. As far as I can see, only one thing can resolve my predicament: a market crash.

Cash from chaos
Global drinks giant Diageo (LSE: DGE.L) is a great example. I have a healthy swig of it in my portfolio, and I really fancy one for the road. But the price of its recent success is high, leaving it on a P/E of more than 17 times earnings and a dividend of just 2.2%. It's a pricey time to take a top-up.

I'm not asking for a repeat of autumn 200; a mere 10% sell-off would bring many of my favorite companies back into range. Is that too much to ask?

There are plenty of reasons for stock markets to take a tumble. European Central Bank chairman Mario Draghi hasn't begun to solve the eurozone crisis, despite claiming to do "whatever it takes" to save the stricken currency. We could still suffer a Chinese hard landing, with the country's GDP growth shrinking again this month to 7.4%. Finally, there is that great unknown: the U.S. fiscal cliff, which could wipe out up to 4% of the country's GDP and plunge the world back into recession. It's looming closer every day.

Oh, and I almost forgot the potential for conflict in Iran.

I don't fancy buying a company at 18 times earnings when the outlook remains so volatile.

Feeling dippy
I could certainly do with a banking stock sell-off. In fact, I think one is overdue. As I mentioned, Barclays is up 50% since July. Lloyds Banking Group is up 35%, and RBS is up more than 40%. The banks are set to remain volatile for some time yet, and I want to buy on a dip, rather than a rally.

Fashion disaster
A wider market sell-off is a better time to buy than a stock-specific slump. Take FTSE fashion favorite Burberry. Its share price recently fell by around one-third, thanks to falling demand from China. But that doesn't make it better value. Mostly, it suggests it was overvalued before.

Burberry may still be a buying opportunity, and indeed the stock has rebounded 15% in the last couple of weeks, but that's not the same thing as a mass sell-off that punishes the good stocks along with the bad.

Bargain hunting
There are other fallen stocks out there. Tesco (LSE: TSCO.L) is down more than 25% this year, taking its yield to a tempting 4.8% and putting it on a forecast P/E of 9.5 times earnings for February 2013. It doesn't need a market dip to look a better value than it is. It just needs a few years to recover.

But if the market fell, it wouldn't be the first supermarket I'd buy. I'd head for the tills at retail rival Sainsbury, which has thrashed Tesco lately. That's a stock I'd like to pluck from the wreckage of a crash.

When I look at the shares I bought on one of last year's dips, it makes me long for another repeat. Medical-appliances company Smith & Nephew is now up 25% since I bought it, giving me plenty of cover against future turbulence. It's been a mixed year for miners such as BHP Billiton, but because I bought it directly after it had slumped 30%, I have been insulated against excess damage.

Crashing bore
I'm not being greedy, and I don't want to tempt fate. What we need now is a good old-fashioned dip of 10% or, even better, 15% -- just as long as it took Unilever, Diageo, Barclays, and Sainsbury with it. Oh, and Vodafone and Scottish engineer Weir Group…

OK. Now I am being greedy.

Eight great shares
Top fund manager Neil Woodford at Invesco Perpetual knows how to keep his cool in a market crash. If you want to know how he does it, then download this free Motley Fool report: "8 Shares Held By Britain's Super Investor." This report reveals the identity of eight shares held by Britain's super investor. This report is absolutely free, and you can order it with no obligation.

Warren Buffett is buying shares, including a top British blue chip! Read this special report -- "The One U.K. Share Warren Buffett Loves" -- while it's still free and available.

Further investment opportunities: