Very good tips Sil!
I've read over them, and I thought that maybe I can contribute?
First off, it is always good to keep in mind that no matter how much time you spend on developing a new character, no matter how satisfied you are with their design/story/personality and no matter how good they might sound/look to others, etc, they are still very capable of becoming just another wasted space in your roster. What I mean is, they don't show up in so much as two roleplay sessions and instead just sit there....rotting...and not be used again. And it IS possible, and probably more likely to happen if you have a lot of characters. Every roleplayer will experience loss of muse (motivation or ideas to write for a character) or become distracted/focused on a certain set of other characters. Your timing for the character might have just been poor IC wise, depending on the events that have already been set in your group's story. Whatever the case, it can happen, and chances are it will.
That being said, also keep in mind that the complete opposite can happen. You might have created a random character out of the blue, with no time to work on his/her back story, personality, whatever. You might take some time to work on their personality after a while, but you don't have any big plans for them. And yet, suddenly, there comes the perfect opportunity for your character to shine, for whatever reason they become tied into some big plot because of one roleplay, and suddenly, woop-dee-do, you're using them often. Now this doesn't happen nearly as often as the opposite, but it can happen.
Next thing. Give your individual characters breaks. Don't stick to one character all day for several days. Mix it up, give your partners and yourself a chance to use different characters and promote some new development between them. You might have a ton of muse for one character, and that does happen, but try to use a character that you would be comfortable using in their place every once in a while. Who knows, you might just write up a really good roleplay using that character. And while your favorite might be a likable character in general, using that same character over and over and over in several consecutive roleplays can create boredum for your partners. This is especially if you're on frequently.
That's all I have off the top of my head.