I bought an iPhone 3G soon after it's release, and it is still in pristine condition. And people frequently ask me "LanceJ, what should I buy for my new iPhone?"
This is my short list.
Silicone Case - under $5
A silicone case protects my iPhone from many hard drops and can keep my iPhone looking new for years. Note that not all silicone cases are created equal - some are flimsy and thin; others are bulky and thick. I like the ones that are just in the middle. I like, in particular, the Eforcity Silicone Skin Case Cover.
Car Charger - under $5
When I'm in the car, I use the Maps feature of the iPhone a lot. And the Maps feature really does chew down the battery life. An iPhone car charger is critical if I'm in the car a lot. I suggest that people buy the GTMax Black Rapid Car Charger.
Spare Charger with Charging Cable - under $15
I find it great to have a spare iPhone charger with a spare cable. I can leave one in my office in case I forget to charge at home. And if I lose or break the original charger or cord, I have a spare. I got a deal with this USB Power Adapter with Data Cable.
A Bluetooth Headset - $20 to $60
The iPhone comes with a decent wired headset, but I love my bluetooth headset. That way I can just leave my phone on my desk and walk around without carrying anything. I can even do the dishes or other things* while on the phone.
I use a BlueAnt Z9i headset with my iPhone. The Z9i fits my ears perfectly and it sounds great. Then again, almost any Bluetooth headset will do. I've also successfully used a Jabra BT125 and a Plantronics Voyager with my iPhone. They also work well, but they don't fit my ears as well.
Unfortunately, I think a Bluetooth headset is one of those things that is tough to buy. Since they fit in the ear, they're not all equally comfortable. That's why I like the Z9i so much - it fits my ears perfectly and it sounds great to me.
What I don't like
I don't like so-called "screen protectors".
These flimsy pieces of clear plastic that cover the screen just get in my way. I question their utility - I suppose it is possible to scratch an iPhone's screen without smashing it, but it seems to very rarely happen. My children have an iPod Touch, and even they haven't even managed to scratch their device's screen yet.
I don't have a screen protector, and I don't plan on getting one. But they're inexpensive enough that it is reasonable to buy one and try it out.
Tips for using your iPhone.