Alright, so we've figured out who's playing with us, which ruleset we'll use, what type of campaign we're running (homebrewed or published) and we've got a decent idea on how we'll handle the thick stack of rules. Now its time to get organized (and yes, I realize we haven't actually talked about adventure design, etc. yet, there's a reason for this).
Organization is probably among the most valuable skills for a DM. Arguably, its as important, if not more so, than creativity since if you're creative juices are not flowing, you can always run published adventures. A good DM will be organized throughout the campaign, starting with initial planning stages and running through each individual session. An organized DM for instance, will remember that the mayor of Thornbridge (a small village that the DM did not expect the party to come back to) is named Corwin, not Werner. Its a small thing in some respects, but its also what makes your world feel alive. Its generally a safe bet that even on a little detail, if you mix it up when the party encounters it again, one of your players will notice. Mixing these details up won't ruin your campaign, but it will detract from it.
Write it Down
What this means is that you need someplace to store all of these little bits of campaign information that may or may not come up again. There are numerous ways of doing this of course, from simply printing out a campaign notebook to going electronic. My recommendation would be to go with a wiki of some sort. Campaign wiki's allow you to link entries together while also using tags to pull up those entries that relate to a certain aspect of your world all at once. The Wizards site now offers a wiki as do some other internet sites. Personally, I use Obsidian Portal, in large part because it was up and running before WotC's site.
The nice thing about wikis is that they also allow your players to contribute to the information. Whether it be in the form of personal character journals or just notes about the world. Whenever your players contribute, make a note of it because invariably, they'll mention what they found important. They won't comment on the innkeeper if they don't care about the innkeeper. Keeping note of what they comment on will help you to design adventure hooks later on. If they care about the innkeeper, then they'll jump at the hook that suggests she's in trouble.
The downside to the wiki format is that if you do not own a laptop, it can be difficult to access the information during a session should it become necessary. The answer to this of course lies in preparation and knowing the area that the party is in, but every so often, they will throw you a curve ball and go in a completely different direction from what you anticipated. If you are concerned about this, you can create a notebook that you print out all your information in, organized by locations, etc. With this, no matter where the party goes, you'll be ready.
Being organized during the session is also important. You need to have your session plan (i.e. the adventure for that day) easily accessible (whether its in your head, in an outline, etc.) This prevents you from having to pause to remember what's behind door #2. I find it also helps to have the monster stat blocks that are going to be used in a session close at hand.
In the case of a published adventure, all of this will likely be ready for you in the adventure (though not all older adventures provide all of the monster stat blocks). If you are going with your own adventure though, you'll need to prepare this yourself. I like to give myself a map and an outline of the adventure handy. This is particularly helpful should your players go in an unexpected direction. As for the monsters, there's a couple of solutions. First, you can tab your monster manuals with post-its on the entries for each of the monsters. Even easier though, if you're playing 4th Ed. anyway, is to simply printout the entries from the Monster Builder. That way you may be using monsters from 3 different books and a dungeon article, but you have them all paper clipped together in a few simple sheets.
Despite everything I've said above, its important to remember that everyone has their own methods that work for them. Just because it works for me, doesn't mean it will work for you. Just keep trying different methods til you find something that works.