I'm currently planning an ongoing campaign (no, I don't have a name for it yet), which I've been mulling over in my head for some months now. I've DMed 4E before, but just a few times and they were always one-shots. Of course the players were new to the game and it was my first time DMing 4E (despite the fact that it's vastly easier to DM than 3.x, it was still a new system). One thing I noticed was that because combats tend to include larger numbers of enemies (often different kinds of enemies), I was flipping through the Monster Manual a lot. Even with pages bookmarked, it still took some time away from combat (not a whole lot, but the DM already monopolizes a lot of the combat time simply by virtue of controlling so many combatants!).
So as I'm planning this campaign, I think that I'm going to record the stat blocks of certain enemies on index cards. I've already created one custom monster, and statting it out on a card was the obvious choice since it takes up less space than a piece of paper (this is important at a crowded D&D table), and can by placed on a MM page without obscuring another relevant stat block. So while I'm not going to get rid of the MM for in-game use completely, I will keep a cache of stat blocks for the more common/recurring monsters that I plan to use. This should reduce the amount of time I spend flipping between pages in the book, as I can just glance down and all (or at least most) of the relevant stat blocks are right in front of me. Will this add more time to my planning? Sure, a little bit. But I'll likely only need 2-4 monster cards per session, which is only a couple of minutes spent copying them from the MM. Well worth it for the convenience, in my opinion.
While I'm on the subject of index cards, I should also note that I've found them invaluable as a player. The power system in 4E is nice in terms of balancing the classes, but it does result in more things to keep track of. Add to this the fact that on a character sheet that's printed double sided (once again, my preferred method for reducing clutter at the table as well as saving trees), the power list is on the back whereas most of the relevant information is on the front. And of course, the power list doesn't include space to write what your powers actually do. So you can either a) copy them down somewhere, b) buy WotC official power cards, or c) flip through the relevant PHB or __ Power book every time you want to use a power. Obviously "c" is a poor choice, since it increases the number of books at the table and/or may result in people needing to frequently share books. Option "b" can get very expensive, since a set of power cards is $10, and that only includes cards for one class from one book. So if you have a Fighter, for example, you'll probably need to spend $20 on PHB Fighter cards and Martial Power Fighter cards. Throw in an extra $10 once Martial Power 2 is released. Not fun, especially since you'll never even use the vast majority of the cards (unless you're an experimental player and retrain frequently and/or always play Fighters of different builds). Option "a" becomes so simple and convenient by comparison that I sometimes wonder how on earth WotC can even manage to sell power cards!
Index cards are a perfect match for option "a" (you probably saw that one coming). They take up less space than a sheet of paper with your powers written on them, and they decrease the amount of bookkeeping you need to do on your character sheet. For example, once you use a Daily power you can simply turn the card over and move it to the back of your stack. After each extended rest flip all of your expended powers back up. Simple. I would also like to note that for power cards, which tend to have less text than monster stat blocks, I cut 3x5 index cards in half. That part's personal preference, but when I first tried out power cards I would slip them into the sleeves of a three ring binder style plastic trading card page (only 1/2 cards would actually fit). This way I had all of my powers visible, and I could use a dry erase marker on the plastic sleeve to note expended Dailies, etc. I abandoned this system early on because having a stack was easier to deal with, could still be used to manage Dailies, and because the dry erase marks would sometimes smudge off if I leaned on the page.