Talking of attributes, they seem to be a fairly underused feature of Objective-C. I would guarantee that we could all take advantage of them to add more context to our code to help the compiler do its job. Nolan O'Brien gives a thorough run down on what they are and how they can be used today.
Since iOS (and more specifically since clang) the pace of change in the Objective-C language has really picked up. This is great news but have you kept up with all the changes? This new document from Apple on modern Objective-C details all the enhancements and will bring you up to speed.
SSL pinning is the technique of guaranteeing that an SSL connection you have is with the server you intended it to be with by including a copy of the certificate inside your app and checking it when network requests happen. Unfortunately this is broken in AFNetworking 2.2 but Eric Allam has a solution.
Ah singletons, ensuring that we still have something to argue about when tabs and spaces merge into one ultimate whitespace character. In this post (and the follow up, which you should also read) Brent Simmons gives a few options to manage objects that might tempt you towards global state. I can't say I agree with all of this but it's always good to know your options.