There are two broad categories of "fix" available to you here. The first is to work on preparation, the second is to work on failure recovery.
From your question, the biggest issues you have are leaving out details in descriptions, and ad libbing NPC dialog. Your preparation should shore up those weaknesses:
Organize your notes such that all of the descriptive portions of each area/character are in one place. When you go to describe something, refer back to your notes whether you need to or not.
When you know that an NPC is going to be interacted with, prepare a list of talking points for them. In addition, try to take note of general opinions and moods the NPC has. Is he a coward? Does he like having adventurers in town? Is he big on politics?
For NPCs, choose one or two "gimmicks" (the dumb one, the hick, the snob, the greedy one, the one who doesn't like his landlord) for the NPC and work around those. For locations, choose a few important and awesome details to include.
If this is a recurring problem for you, you should also practice. General advice about public speaking applies here: Practice until you know what you need to say, but don't get hung up on precise wording (that tends to make you sound like you're reciting).
Practice delivering NPC "set piece" speeches, or describing locations. If something doesn't sound right, tweak it until it does (this applies to the "real world" too: If one of your stories fell flat, try figuring out what went wrong and polishing it up).
During this practice, make sure you're actually speaking out loud. There's a huge gap between what you say in your head, and what comes out of your mouth.
No matter how much you prepare, some degree of failure is inevitable. Learn to smoothly deal with and recover from mistakes.
Mistakes are generally going to fall into two categories:
For many mistakes, just keep going. If the mistake or omission isn't critical to what's going on, don't worry about it. Try to work it in later (if it's an omission), or keep going forward using the mistake as truth. Interesting things can happen when you push the boundaries of what's unimportant.
If the mistake is critical, there's not much you can do other than calmly asking your group to hang on a minute while you correct things. Don't get frustrated, and don't treat it like a big deal. Verify your facts, add the missing information, and then ask the players if that would change anything they did.
In a few cases, something important will be missed in a way that it's impossible to roll things back (an ambush is sprung, or you don't think about it until much later). At that point, I would recommend erring on the side of your players, and taking the first opportunity to go back and figure out what needs to change. In particularly extreme cases, you may need to end a session early, or call a break while you sort things out.
The key is not to get flustered. Mistakes happen... As long as you stay calm, and give the mistake the appropriate amount of attention you'll do fine.