It's happened to all of us, but it's especially noticeable (I would believe) in the department of 1×1 RPs: you can't think of a reply to write to a post. Your partner(s) have written one helluva post, and you begin typing a reply. However, you stop halfway through, thinking, "No this is too cliché!" or "How would this character respond as I've written them/in canon?" You try multiple times but get stuck in a rut and wind up getting so frustrated that you facepalm… with your keyboard.

Funny mental image aside, I'd like to hear how everyone keeps the creative juices flowing, or what gets you out of a rut. As crazy or strange it may sound, if it helps continue a RP I would love to hear about it and discuss it!

It depends regarding the roleplay's genre, music of an undecipherable language helps (like german in guilty crown's bios theme to scifi scene). I would say japanese music is good for this but personally, since it is one of my two native languages, it is not as effective on me.

Though instrumental music helps too.

There's also sleeping, because dreams get very vivid if the mind is fretting on something and the rest is undisturbed. I do not advocate drugs but burning/smouldering dried basil leaves before rest at night seems to increase the chances of dreaming, basil is not a drug anyway. But then again, I'm someone who thinks I can surf the web and wikipedia things in my dreams as long as I rest near an unprotected wifi. So my words may not be entirely sane.

Music, sleeping, doing something else, consuming media relative to the themes of the RP, not roleplaying for a few days to a week are potential alternatives.

I find that just because a post is long, doesn't necessarily mean it warrants a huge reply. If I write a 2k word inner monologue while my character more or less ignores everyone around them, I'm not giving anyone anything to work with. But just by having my character ask a simple question like "So how did you end up in this mess?" I'm inviting you to write paragraphs worth of dialog and thoughts. So it might not be the best idea to constantly try to match another writer's length. And it might not be your fault you have nothing to write about.

As long as someone engages me, it's not difficult to keep the creativity flowing. Typically when I get stuck in an RP, it's because either I or someone else wasn't engaging enough. but if you feel you have plenty to write about and nothing you write satisfies you, maybe you just have writer's block.

The largest portion of advice I can offer is reading and rest. It is no secret reading and writing are tied hand in hand, with one not only generating inspiration for the other, it keeps the metaphorical wheels turning. It is not wholly passive activity, especially if the content is deeper than say, just a story. While avoiding writing for a time, perhaps a day or two at most, reading taking up its place more or less restores the reserves, as long as the reading is intellectually stimulating; reading a tabloid is not going to be as helpful as say, reading a psychology or philosophy book to help understand and formulate a character's mind and its motives. This element and approach I have found largely successful, namely just before sleeping as well which leads me into my other point.

Doing nothing but reading just before sleeping is a fair way to unwind the knot that is the mind in regular activity, especially creatively. I say this because what you take away from the reading will become part of the unconscious to come when you sleep. You begin formulating information to later extract and use from it in the next day. It might be invisible, effectively, but it is there in the underlying thoughts and feelings, that when you do return to writing by the third day in this example, you have had time to create those thoughts and can now tap into an underlying reserve that was not or might regularly not be there; it is free no less, a consequence of reading before sleep. Granted you do not want a page turning book, the sort you get too invested in, but something relevant and building in foundation for the story or character.

As for the rest itself, try to make it actual rest, a full eight hours uninterrupted in a dark room. I know for a fact a surprising number here do not sleep as they should or in the right environment, but for most that does nothing but harm one's actual writing ability because the mind is not afforded its time to successfully draw down. However, those are my suggestions for pulling one's self out of a rut as it were. Having done this for some time I have not felt myself in a place where I could not write due to a lack of sufficient creativity, or so I felt.

Well, not just reading and rest, but also finding an excuse to go out and do stuff.

Personal experiances beside a campfire, sleeping in tents, pulling trucks out of drainage-ditches, topping-off blown transmissions (adding enough fluid so it can limp to a shop), and generally every odd little encounter I've had actually comes in handy in RP settings.

Heck, walking to school, and sparing a few seconds to help push a single mother's car clear of a berm of snow in an intsersection during a blizzard can, all of itself, lead to an RP.

Learning how to paddle a canoe without splashing and walking quietly on dried leaves in the woods was more-or-less the same. A very stiff scolding on the first mistake, followed by things getting progressively meaner as mistakes piled-up. Not being able to get within five feet of my brother w/o being noticed during hide and seek was worth a few dislocated fingers.

I’d say one of the most helpful strategies is giving yourself a root of familiarity to the character. Even if you’re not an alien or pirate or post-apocalyptic survivor, perhaps you do know how loss can obscure perception or the larger-than-life feeling that comes with going somewhere new. Those bits of experience or emotion you know personally can be manipulated and suddenly you’re imagining a world that feels compelling.

eating the souls of more creative people.