I love running what I call 'jazz band adventuring'; it gives me and the players a great deal of flexibility and lets the stakes be really, really high without running the risk of derailing the campaign.
But it also means that the players are rolling up new characters relatively often.
I have a series of things I do for new characters joining the campaign. I make sure to sit with them and work up an outline of a backstory. They flesh it out, but I collaborate for things like home village name and location, family background, etc. I will also often work through initial gear and tell them to make sure they do or don't get certain things. For the rest I usually wait until the characters are actually about to be played (the day before a session, usually).and then give them:
1) Print out a list of what they know.
This has three main sections; facts they know; PCs and NPCs they know; rumors they've heard. If the player's backstory needs certain details from me, I include those, too. For example, my wife's barbarian character Brigid is from a clan in the middle of a feud, so I included the name of that clan.
2) Give them a list of any 'extra stuff' they start with.
This can be all over the map; fighter with the secondary skill of 'farmer' might have extra rations from his mother; a thief with a backstory that he works the streets as a pickpocket for cash might have a specialized tool or two; a nobleman might have a silver-accented saddle and a valet. Once (working with the player) the half-orc fighter/thief fresh from an orphanage started with the flu!
This isn't a lot of stuff and it doesn't take much time or effort, but it pays off a lot, especially since I know their backstory.
Then, before the game, I also add them to my character roster. This includes the usual stuff, like name, level, hit points, stats, etc., but also a sketch of their backstory, the list of what they know and their extra stuff, and such. Last but not least I keep my own notes on major magic items, big adventures, and my behind-the-scenes stuff, as well.
Again, all of this takes maybe 30 minutes, total, per new character but it pays big dividends! The players feel plugged in to the campaign and I can keep track of plot lines much better.