1) Make sure it has an Apple logo on the back and is up to date. I'm serious on this one. Too many Android phones don't get updated by the carrier and that's why I'm not a fan. Yes, if you have the latest phone from Google, you are fine. From another manufacturer, very questionable. The sheer number of Android phones which have connected to my open research WiFi networks over the years and exposed some secret is just tragic, from user PINs thanks to a carrier installed warranty app to e-mail passwords thanks to broken Samsung KNOX TLS middling implementations.
2) Shut off all background activity from apps when not on and in front of me: settings -> general -> background app refresh. Slide that one to off for everything.
3) Turn off WiFi and Bluetooth.
4) For added paranoia, put it in airplane mode when not being used.
5) Make sure it doesn't have any information or accounts on it which I'd not like to be made public.
6) Back it up.
7) A quick audit of apps I'll be using at the con to ensure they are reasonably secure on the wire by using working TLS exclusively. Yeah, very few people will ever do this but thankfully 1-6 should be sufficient.