(MONEY Magazine) – If the stock market's ups and downs are making you seasick, why not put a buck or two into assets that you can at least have fun with -- like a Shirley Temple doll or Pillow Speaker radio? While the idea of sinking good money into such trivia may seem odd, it sometimes turns out to be rewarding -- as the prices on this page show. The reason is that these items, and many others as well, fall into that shadowy category called collectibles -- objects that are too old or too quaint to be commonplace but not yet old enough to be antiques. How can you tell a collectible from an antique or plain old junk? First, by age. An antique is usually older than the turn of the century (although some things, cars among them, are called antiques if they date from World War II or earlier). Telling a collectible from junk is a little harder. Age is part of the story: professional trivial pursuers say that some of the clutter in attics, basements and garages starts to become collectible when it's about 30 years old. ''Three decades are enough time to throw things out, to forget the forgettable,'' says Harry L. Rinker, author of How to Make the Most of Your Investments in Antiques and Collectibles (Arbor House, $17.95). But equally important, a collectible has to have a certain charm -- often a nostalgic one -- that makes people treasure it. That's why the 30 years are important: that amount of time allows children to grow up and become nostalgic for the toys, clothes, furniture and gadgets of their childhoods -- which, in turn, engenders a desire to buy these things and pass them along to posterity. Some objects transcend the 30-year rule. You won't have to wait that long to cash in on your 1965 Ringo Starr bubble bath. Current price: $85 a bottle. Elvis items have had a similar run-up (see MONEY, August 1987). And certain general categories, such as dolls and jewelry, are perennially popular. But investing in collectibles is always something of a gamble, even with such blue-chip items as stamps and autographs. The value of top-quality stamps went up 15% to 20% this year and should keep rising, according to Lewis Kaufman, editor of the Brookman Price Guide to United States Stamps. Autograph prices were also up about 20%, says Charles Hamilton, a New York City autograph dealer, but may slump next year ''as people retrench after the stock market crash.'' With price movements like that, says Lyndi McNulty, a car collector and curator of the Carroll County Farm Museum in Westminster, Md., the safest strategy for investing in collectibles is: ''Be sure you love what you buy, because you may end up having to live with it.'' Something you buy for fun can, of course, turn into a treasure trove. The advertising memorabilia of Ralph and Terry Kovel, authors of Kovels' Guide to Selling Your Antiques and Collectibles (Crown, $9.95), assembled over 30 years from flea markets, auctions and store closeouts for $25,000 to $50,000, is now worth more than $250,000, guesses Terry Kovel. Says Judy Blakely of Old Print Gallery in Washington, D.C.: ''Advertising posters that were selling for $40 to $50 just 10 years ago are now going for $1,000.'' But if you are thinking of selling your advertising posters and fleeing to Brazil, beware: if the market is small for your items, you could be forced to sell to a dealer at wholesale prices. Dealers commonly pay only about 25% to 50% of an item's retail value. The best way to get higher prices is to sell directly to a collector, usually by placing an ad in a special-interest periodical such as Antique Trader Weekly (P.O. Box 1050, Dubuque, Iowa 52001; 30 cents a word, $5 minimum), or by writing for The Guide to American Clubs for Collectors (P.O. Box 25763, Colorado Springs, Colo. 80936; $3), edited by Ed Love. True to the 30-year rule, the big hunting ground for '88, say many connoisseurs of collectibles, will be among holdovers from the 1950s -- particularly toys, furniture, clothing, posters, TV and advertising memorabilia, and cars. As an extreme example, Love cites a man in Los Angeles who collects the isolation booths from '50s quiz shows (he now has seven). But even for more conventional items, Lyndi McNulty says that ''you can still get in on some very good buys at garage sales and thrift shops because the value isn't fully recognized yet.'' To pick a good '50s collectible, look for anything emblematic of the period -- colors such as chartreuse and bright red, what experts on the period call the ''sensitive line'' motif of curling thin lines, and anything in amoeba shapes. Subject matter is important too: souvenirs of the rock-'n'-roll music, the cartoons or the TV game shows and sitcoms of the day should rise in price. One thing stands out from the decade: as the businessman said to Dustin Hoffman in The Graduate, ''Just one word . . . Plastics.'' Plastic radios, plastic lamps, plastic jewelry, plastic toys, plastic kitchenware, plastic cups and saucers. And of course, plastic pens, plastic salt and pepper shakers, plastic cameras and plastic clocks -- plastic doodads of almost any type. Plastic radios will do especially well in '88, says NcNulty. ''You can still find nice ones worth $75 at tag sales for 75 cents.'' The Pillow Speaker radio pictured on page 65 was sold to hospitals so patients could deposit some dimes and have Rosemary Clooney lullaby them to sleep. But McNulty cautions you to look for a radio in good to mint condition. And never buy anything that you, as a collector, wouldn't put in your living room. If you won't show it, chances are no one else will either. Result: no resale value. Plastic dinnerware looks hot too, especially that made from melamine, a durable plastic also known as melmac. McNulty, author of the dealers' bible, Price Guide to Plastic Collectibles (Wallace-Homestead, $17.95), says melmacware designed by Russel Wright, whose ceramic dinnerware is included in the collection of the Museum of Modern Art in New York City, will command the highest prices (around $5 a plate) but will still be good buys. Shirley Temple plastic dolls are already very popular and should continue to appreciate beyond their present peak value of about $250. Even the humble razor and brush holder (shown on page 65), originally a promotion giveaway, is now worth $25. Plastic toys are also expected to rise in price. Look for vinyl figures of cartoon characters like Olive Oyl by Dakin, a toy company. Dakin's Mickey Mouse, Donald Duck and other Disney dolls now sell for about $27 to collectors but may turn up at tag sales and thrift shops for less. Toys made by Hartland, a Wisconsin company known for baseball and Western figures in the '50s, are a good bet: the firm's plastic Roy Rogers and Trigger figures go for $75 each. And on the unfortunate chance that you snap up some dandy plastic ice crushers or I Love Lucy lunch pails and still find yourself unable to get a good price later, take heart. As Harry Rinker points out: ''In just 12 more years it will be the 21st century, and then everything from the 20th century will take on an instant antique aura, with matching higher prices.''