Just to continue one aspect of postmodernism mentioned above, Milan Kundera does love to intrude into his work, as mentioned. He believes in mixing fiction and nonfiction, as well as mixing the arts. Thus, he discusses philosophy and classical music within the text of some of his fiction. Another aspect of this is that he is blatantly and obviously creating the stories he writes. He completely rejects fiction's attempts to be read as real or actual. His stories are fabrications, but, he insists, readers suspend their disbelief just as readily when they read his fiction as when they read fiction that pretends to be real. The idea for a narrative may come from something such as a gesture, for instance, and he'll tell the reader so, but the narrative is still readily accepted by readers and can be just as, if not more, enjoyable.
I've noticed lots of manipulation of time--flashbacks, flashforwards, time travel, and multi-plane existence. It makes for fascinating stories, but it also makes the reader/viewer really have to pay attention to stay in the loop. Quinton Tarentino movies are some excellent examples!
There are many different answers to your question, since the 20th Century spans the Modernist and Post-Modernist periods. Modernism began in the early 1900's and lasted through the 1950's. Modernist literature introduced such techniques as stream-of-consciousness (such as Virginia Woolf's Mrs. Dalloway) and cinematic description (like the scenes in F. Scott Fitzgerald's The Great Gatsby). Another technique of Modernism was the fractured narrative, which mimicked the visual affect of cubist painting. Joyce's Ulysses is a great example of this.
Post-Modernism began during the social unrest and upheaval of the 1960's, and is marked by the blurring of boundaries. For instance, Kurt Vonnegut and Milan Kundera often wrote themselves into their own books. This is a technique known as "author instrusion." Another technique that is seen often is the prose poem or flash fiction. This is not quite fiction, not quite poetry, but both simultaneously. Russell Edson and Italo Calvino are masters of this form. The effect of cubism on a plot is extended to time as well, which can be seen in Toni Morrison's Beloved, where the past and the present exist simultaneously.
There are many other techniques, but this list should get you started.