The crucial decision is often made just days before the event, and we know it's a crucial decision as one of the most popular questions asked on the red carpet is, 'What are you wearing?'.
A model tries out one of Samora Olayan's Oscar creations
The cameras pan up and down, soaking in the complete image before fashion pundits make their verdict.
So the pressure's on to look your best.
It doesn't exactly boost one's ego to be labelled a fashion disaster, so that's why many stars turn to top designers for fashion advice.
Samora Olayan is an LA-based designer who's run a boutique in West Hollywood since 1994.
She's made clothes for celebrities including Demi Moore, Teri Hatcher, Eva Longoria and Paula Abdul, as well as for regular women.
She's currently in the middle of putting together her spring 2009 collection and has based a lot of her designs around floral and polka dot patterns.
The Egyptian-born designer dished out her top tips for Sunday's red carpet:
Wear some pastel colours that are romantic and calm, like pink or purple
Make sure you wear something comfortable that compliments your body shape
Finish off the outfit with some vintage jewellery and red lipstick
Although Samora believes that feeling good is very important for a woman especially, she explained that not everyone gets it right.
"Gwyneth Paltrow's last outfit on the red carpet didn't fit her properly on the bust," she said. "It looked awkward but she usually gets it right.
"And Celine Dion also looked not great in a white dress. It was showing too much skin and leg and it was just not right for her."
So far, Samora's put together five dresses for the big day.
They include a brown halterneck gown with a sea blue trim, and a red dress with a dark blue eastern pattern.
One of Samora Olayan's sketches for a £3,000 dress
These outfits might be glamorous but they don't come cheap - the price tag is around £3,000.
Samora told Newsbeat it was top secret who might wear the dresses on Sunday.
Some fashion experts in Hollywood reckon this year is going to be different on the red carpet.
With the US in an economic crisis, many think stars will tone down their outfits to reflect this.
Newsbeat asked Samora if she thought it was insensitive to dress celebs in costly gowns when ordinary Americans were financially suffering.
"The Hollywood world is based more on a fantasy than a reality," she replied, "and seeing the stars wearing beautiful dresses, it lifts up people's spirits and makes them look forward to a better time."
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.