My only piece of advice, from my limited experience, is to try to spend a lot of time imagining yourself living the life of your character. The challenge level is often directly proportionate to how well you can relate to and think like your own character.
Knowing your character is really important. Before I write a new character, I write an entire biography and usually will keep a "journal" in that character's perspective for a couple days to build up some backstory and get a feel for how the character acts.
Important to notice too is that every character should have a slightly unique vocabulary. For example, in my fan fics, Nick <i>very rarely</i> will use a big word. When he does, it's usually close-but-not-quite the word he was <i>trying</i> to use, or else someone makes a comment about him using it. Unless I'm trying to make a different perspective on him (like in Time Watcher, where he's actually much older than he appears), that's how Nick rolls. Nick tends to talk more in a sentence, too. He'll ramble. AJ uses a lot of swears - fuck is his favorite - and Brian tends to talk dorky - "ohmigawsh", "heck", etc. Howie and Kevin are my big-word users, because they're older, but Kevin uses more in a single sentence than Howie will use in a whole paragraph.
Stuff like that is really important to capture because no two people - even people who spend a lot of time together - will speak exactly the same way.
But then again, too, because they DO spend a lot of time together, they're also going to speak a little similarly. There's going to be certain phrases and figures of speech that they share - fake words they both use - slang they've picked up from each other, etc.
Tiny little things like that add IMMEASURABLY to the reality of a character whether it's first OR third person. But inner dialogue is really important to pay attention to when you're doing first person, because you HAVE to stick to the same speaking style as they speak aloud with throughout the whole story or it looses its authenticity. Ie., Nick can't be dumb when he talks, but a genius on the inside - unless you have a REASON for that - like he's hiding his inner intelligence..
. *snickers*
That said, I find first person narratives easiest because it lends the most creative opportunity. You guide your reader through this person's world and make them see from a new perspective. IMO, that's the true purpose of literature of any kind.
