There's more to finding an A-list hairdo than simply plunking a copy on Woman's Day in front of your stylist and demanding "That one!". Consider these factors...
Hair Type
Above all else, this makes the difference between a hairdo that actually achieves the look you want, and a disaster. Basically, hair comes in types, and not all the product or styling in the world can make your hair behave too differently from its natural type. If you have Gwyneth Paltrow hair, don't even try for a Natalie Portman look—it won't work.
Consider the three aspects to hairtyping: straightness or curlyness, thickness and texture. Is your hair stick-straight, just wavy enough to have a little body, or do you have full spiral curls? Is your hair thin, medium or bushy? Fine, medium or coarse? Now go through your mental portfolio of celebrities with great hair and ruthlessly eliminate all those who don't match your hair type. Thick, bushy hair won't hang in the same way as fine, thin hair. Savvy celebrities work with what they've got, and so should you!
Face Shape
Second in importance to hair type is facial structure. Celebrity hairdos are usually designed to highlight or disguise certain features. The easiest way to tell the agenda of a hairdo is to compare photos of your chosen hair idol sporting different haircuts. Does her chin look bigger with this style? Does it de-emphasise her forehead, or emphasise her eyes? If the cut highlights everything about your face you want to conceal, avoid it. A cheekbone-enhancing cut won't do you any favours if you have chubby cheeks, so find a similarly chubby-cheeked celebrity with great hair instead! A fun, quick way to find celebrities with similar face shapes is to use a face recognition program.
Wearability
Celebrity hair looks great because it can afford to. Many celebrity looks are incredibly labour-intensive to maintain. Be realistic. How much time and money can you afford to spend on this cut? Will your budget stretch to root touch-ups every three weeks? Can you spend 45 minutes blow-drying every morning? Does product make your hair look greasy?
Next, consider living in this hairdo day in and day out. Does it lend itself to a variety of styles—quick morning ponytail, business casual and out at a cafe? It pays to find a few different candid shots of your celebrity (this could be one of the few semi-legitimate uses for the paparazzi). If her hair looks half as good picking the kids up from preschool as it does on the red carpet, that's a good sign! And of course, avoid any look which either covers an eye or requires holding the head at a certain angle and pouting to look good!
Fashion Reviews
This isn't as important, but it might pay to check out what everyone else thinks of the hairdo first. If the look you love has been met with shrieks of 'What has she done?' (Renee Zellweger's much-panned bob at the 2008 Academy Awards springs to mind), you might wish to reconsider. After all, if you've followed the above rules and chosen a celebrity with similar hairtype and facial structure to you, chances are a 'do will give off a similar impression on you as it did on her.
On the other hand, if you love it, go with it! You can hold your head high knowing that even if it's a disaster, at least it's a disaster of A-list proportions.