For you players thinking of running Nami due to her appearance at MSI finals...
My best advice is probably to not use your (Q) Aqua Prison wildly; it's best used when your target is either focused on a target and not moving much or at all, or if it's already CC'd, including slows.
Most people react to Nami one of two ways; A) When in range, they retreat directly away from her to avoid the Q, or B) run directly at her going for a kill.
A) This is most common in lane. Aiming just at their back will catch them more often than not. They either don't move and get hit, or they try to retreat and get hit dead-center.
B) If they are fed and/or they are melee, they frequently just make a beeline directly at you. They are really easy to predict, just aim in front of them and adjust for their speed. If they reverse to dodge, then you still bought some time. If it's something already on top of you especially like Yi or Ekko you can toss it right on top of yourself while they are hitting you (Yi is about the worst opponent, you have to wait for him to Alpha, or nail him at max range before he can).
Get used to the time Zhonya's, Bard Ult, Teleport, Panth Ult, Zil Ult, TF Ult, GA, and any other similarly telegraphed abilities take to finish. You should be able to hit every single one of them so that they can't even Flash out. TP is probably the easiest; listen to the audio cues.
If any enemy likes to or has to sit in one spot to do something, watch for it diligently. Pantheon has a whopping 3 abilities that guarantee you can Q him. Many champions have Ults with channel times, none more obvious and easy than Malz, Warwick, and Karthus.
The above applies to your allies too. Not only is Pantheon an easy enemy, he synergizes with Nami as well! Allies with guaranteed stuns are the best, so when they are coming in be sure to save your Q until they lock down the preferred target.
You'll also want to combo Q > R often. Tidal Wave is easy to hit, so can guarantee you to an Aqua Prison; the more, the merrier.
or not 
, scorpions and spiders and birds and bears and centaurs can wear the same shoes as everyone else, tentacle monsters and dragons and even trees and rocks can buy shoes, but one particular snake now can't. It really highlights the arbitrariness of an item system where, e.g., buying a big sword makes your arrows, gunshots, or even spitting at people do more damage.
and the item I always wish I could fit into healish support builds but usually can't, 
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to open for CDR/the athene's passive, and then stack AP unless the team needs something specific (since I'm CDR capped, more AP is the best bet unless I need a specific defensive stat). That build can vary wildly, but in general that's what I go for. I'm pretty greedy with my support builds though...