Take time. Be prepared. Revising whatever you have written/wanted to say will help.
 
In the beginning, write whatever you want to say, by writing you will actually see the words you want to say and will realize whether you really want to say that. Even if it is a debate, writing the points in gist is important. Revising it will make you feel how it will be when you present or does it needs some modifications.
 
Think if somebody is presenting something to you (what you have composed), will you understand what they are trying to convey? is it short and sweet? I mean, even if it is logically correct, if it is taking long time to come to the point, people will be bored.
 
If the first thought that is coming to your mind is not accepted by many people so far, don't tell out the first thought instantly. Just give thought for a moment, rephrase it in simpler words what you want to say (can be in your mind) and then present it to others. People nowadays want everything simple.
 
With respect to code, we have something called design, which is nothing but writing the steps in plain English. First of all come out with a design (can be in the interview itself, it will impress them even more). It should be convincing for you that every step you have written is of some importance. When you translate it into code, make sure you don't write unwanted steps (something which will change the requirement/design with additional steps).