It's a combo of genetics, skincare, and lifestyle. Olivia's main job is to go to parties and be a socialite, despite whatever the City tries to portray. She doesn't have to stress out about money/school/work and has enough money to go to a top dermatologist, as well as the time to get weekly facials or quick-fix injections during a breakout. Not to mention, she is always wearing a ton of make-up in every photo so even the tiniest flaw is masked.
Most people do not have time/resources to do this... it's hard enough fitting in a regular exercise routine, let alone making sure each hair curl is perfect, make-up is flawless, nails are not chipped, outfits are perfectly accessorized, etc..