Let's face it, there are some characters with reputations. I'm not talking about In Character reputations, although many of them will have one of those as well. I'm talking about those characters whose threads get stalked, even when they're not doing anything all that interesting.
Whether they're a character we love, or a character we love to hate, they're memorable and most people want to have a character like this. However, it's not easy to do, and there can be a lot of pitfalls along the way.
Know where they start on their journey
In the Harry Potter books, Harry starts his journey by learning about the Wizarding World. His life up until that point had been dull and repetitive.
You need to introduce your character at the point when things get exciting. Start them too early, and you'll get bored. Start them too late, and you'll end up padding the character's history. Your character's history should always be more boring than their threads.
Dedicate yourself to this character
On any site I've ever been on, the number one deciding factor in making a character memorable is how long that character has been on the site and how much they post.
It doesn't matter how interesting or boring your character's exploits are if everyone on the site has seen a thread they've been in, or even better, been in a thread with them (so post open threads).
When you see people like me and Jack and Addie with many characters, it might seem like the number of characters a person can have is a bit of a competition, but I'd estimate that only about 3 of my characters are truly memorable and they're the ones I'm dedicated to.
Write for yourself
Don't try to write a character who's cool, or who you think other people will think is cool. You can't be dedicated to a character just because you thought it was a cool idea. Write a character you love.
Let your character stand alone
Your character's most important relationships should be with other player's characters, not with NPCs.
I can't pretend to know why other people do this, but when I was just starting out in RP, I did it because I wanted my character to be perceived a certain way. If that's you, I can tell you now, it doesn't work. Players and their characters will form opinions based on how your character interacts with other PCs because those characters are higher up the hierarchy of the game.
Don't construct your character around their job, their faction or a personal plot
It's no coincidence that most of the memorable characters on PA started out at Hogwarts and became noticeable by participating in site plots. These characters were created as characters first, not to fulfil a role.
Many sites have stringent activity rules, but one of the things that makes this site nice is that we understand that real life comes first. However, that gives us a high character turnover, and other characters who come and go frequently.
That means your character needs to be adaptable enough to interact with other characters outside of their job, faction or main plot. In fact, I wouldn't recommend planning a personal plot at all.
Concentrate on one thing
Your character may have many strengths, weaknesses, talents, etc. These things may come up according to circumstance.
However, your character should only have one defining strength and weakness. This is what your character will be known for. It's how they get that reputation. For example, Selwyn's defining strength is his sense of responsibility, his defining weakness is his insecurity.
Give your character a voice
It's not always useful to "show don't tell" in RP, since we like reading about character's motivations and thoughts. However, it is useful and more compelling to write your character's inner monologue in their voice. It also prevents repetition and kind of posts that read like "Sally did this, then this, then that."
For example:
Selwyn hated the first class of the day. It was intolerably early, directly after breakfast had finished and it wasn't as though he could be late to his own class.
He scrubbed a hand through his hair as he waited for the students to turn up to the creature paddock. Hopefully they would be just as tired as he was and he could assign them busy work for this lesson.
That's not an entire post, obviously, but here's what it looks like when I use Selwyn's voice:
What had he done? Selwyn languished in his chair. This had seemed like such a good idea at the time, and like all his good ideas, he hadn't thought it all the way though. Oh, sure, the animals were well rested. That was good. The students, when they got here, would be well fed. You couldn't go to Hogwarts and not be well fed. Well, not unless you were like Shylock. But Selwyn didn't function that well without an extra hour to sleep in. That was the bit he'd forgotten.
Already you can see how much easier that is to read. It's no longer just a list of one thing happening after another, almost all of it is filtered through Selwyn's perception. Because it's his perception, I'm writing in the way Selwyn would think, and very similarly to the way he'd speak, which varies my sentence length (that's one of the reasons it's easier to read).
It also makes it easier to add in references and character development, like how Selwyn was the one who organised this early class but didn't factor in that he likes to sleep in.
Push conflict in every post
Except for perhaps a starting post (and sometimes, even those ones), there should be some kind of conflict happening in all your posts. Conflict doesn't have to be direct, it doesn't have to be an actual fight. It can be unacknowledged, it can be passive, it can even be the internal conflict in your own character. But conflict is what makes stories, and by extension, RP posts, interesting.
You can tell when a thread is coming to an end because it happens when the main conflict either finishes for now, or is resolved.