Other people's comments and work!
Negative comments when phrased constructively, really help me to improve my writing. Of course, positive comments also really help too.
I belong to two postal folios, one for Fantasy and Science Fiction writers and the other for general fiction. If you're interested in joining us and have a completed book that needs constructive criticism, email me at firstname.lastname@example.org and I'll send you the contact details.
The folios are excellent because the quality of writing is extremely high, often better than currently published books. Everyone has such different ideas and various ways of executing a plot that I feel I've learnt more from these two folios than from anything else, including a creative writing course at college. It's how people write that is so revealing. And when you're reading one chapter of a book every 2 months, you have time to analyse and really think about what the writer is doing.
I also subscribe to Dial 174 a magazine for poet lovers. There's always something for everyone in each quarterly issue and I find it very inspiring to read other people's work. The poetry ranges from traditional iambicpentameter and rhyming couplets to haiku and blank verse. There are also several short stories in each issue as well as articles. I have yet to win one of their in-house competitions though...
I used to subscribe to Scheherazade, when it was still being published. This was, in my opinion, the best small press magazine for fantastic fiction. Not only was the quality of the writing superb, but the illustrations were like something out of an expensive fairy tale volume. Each issue was indeed a magical read. I'm hoping that one day Scheherazade will return to print.
Comments, other people's work, all these things inspire me to write more and to write better. If I don't keep honing my skill it will die.