From a story standpoint, you have, more or less, two choices: Recount what's come before the current book, or embed critical information in the narrative itself. The more realistic it is for characters to talk about past events, the less need there'll be for a summary of events at the beginning of the book. But if you want the book to seem like a grand retelling of important historical events, placing the reader in time might suit your story very well.
By including a summary of previous books before the story starts, the reader will be firmly oriented before starting the current book. However, this also makes it obvious that the book is part of a series, and makes it appear to be less self-contained.
This sort of structure is useful in a series meant to evoke a feel of a chronicle of real events, or to create a sense of being part of a larger picture.
Example: Tolkien's The Lord of the Rings, Donaldson's Chronicles of Thomas Covenant
Embedding critical information in the book itself can be difficult to do well without seeming clumsy. It will make the manuscript slightly longer. It can make the book more accesible, in that one doesn't have to pick up previous books to read it. Common ways of doing this include having the characters talk about recent events, but this can easily become clumsy.
While the risk of losing readers in the plot is real, this can be mitigated by keeping the plot of the current book reasonably independent or at least interesting whether or not one understands the background.
Example: Dan Simmons' Hyperion books, Doyle's Sherlock Holmes
Of course, there are cases in-between these two extremes. For example, Asimov's Foundation books include summaries of the book in broad strokes, then rely on either contextual clues or characters recounting events in-story. This sort of thing is more believable in a context like this - sections take place years or even generations apart. Family sagas, and historical fiction can do this, as can fiction in other arenas where recounting events is common.
And let's not forget the strategy of using a narrator. While this has fallen out of fashion along with the cinematic voiceover, it's still a valid technique - even if it tends to be cheesy and obvious.
Which one of these strategies you use will be determined by personal preference and hoe complicated and interdependent your books are. For example, if the books have independent stories and the plots are fairly simple, making them stand on their own will be less complicated.
Another question to ask yourself: How large is your cast of characters? The larger it is, the more time you'll spend on character introductions in each book. Including a list of characters can be a way around this. If it's fun to read, and the descriptions are brief, it can even add to the book. However, it will add to the "series" feel if you include one.