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.
