Santorini was just one stop on my European summer trip so I was only there for a few days. I stayed in Oia and while its beauty is undeniable, there are a couple of things I would do differently now so here is a short list of my dos and don'ts in Oia. (& of course these are all objective)
Starting off strong:
DON'T go to Oia in the summer. I KNOW this sounds crazy, but if you are like me and you don’t like large crowds packed in to tight spaces, then avoid Oia during summer months. A better time to go would be in the fall or spring months (smaller crowds and better weather).
DO order a greek salad every where you go to eat because they did not fail me once (and its only right considering you are in Greece).
DON'T buy the first thing you see. A lot of the time, the shops will sell very very similar things for different prices so shop around a little until you find the lowest price.
DO get started with your day early if you are there in the summer months and plan on walking around and shopping. Like I mentioned earlier, the summer months are packed with a lot of tourists AND cruise ships. Five to seven cruise ships can arrive in a day and once they all unload, the shops can easily become over crowded. So, to avoid that, get an early start to your day so you can beat the cruise ships.
DO find a place to watch the sunset. Oia is the most popular place to watch the sunset in Santorini so the streets become packed when its time for the sun to go down. There are a couple of very popular sunset viewing spots like Oia Castle, but crowds are guaranteed to be large here and honestly, if you just wander the streets, you will find other spots with only a handful of people and an equally beautiful view.