Tools of the trade
One of the things that will make you look more professional as a scriptwriter is sending out your work in proper script format.
Script format is different in radio, television, film, and on the stage, to allow for the different requirements of each medium. Fortunately we've got a page on the website dedicated to all the different formats, so you can see which is which.
Back in the old days, when beer was tuppence a pint and children respected their elders, you would have had to put in all the margins and spaces using little sliding pieces of metal on your typewriter to set the tab stops. Fortunately, you can now use various computer programs to make your life easier.
First up is our very own Script Smart. Script Smart is a set of templates for Microsoft Word that has the various tab-settings already input. Unfortunately Script Smart is a few years old now, and we don't have the resources to support it on newer operating systems, or to port it to different word processing suites.
If you're looking for something a little shinier, there are commercial screenwriting programs that you can buy. The two best-known are Final Draft and Movie Magic Screenwriter. These have an advantage over standard word-processing programs in that they'll do useful things like remember character names and location names for you.
Finally, there's an open-source screenwriting program called CeltX which is starting to gain ground. It's available for free.
Sadly, no-one yet seems to have invented a writing program for Radio format - so to the best of my knowledge you'll still have to set your own tabs for those.
If you know of a great screenwriting program I've missed - tell us about it in the comments below.
(EDIT: We used to also point people in this entry to a program called Sophocles, which is no longer available.)

Comments
Well... ScriptSmart does radio, so there's that.
Yes, those versions of Script Smart are a bit old, but they still work fine on Windows XP, Office 2003 (though not tested for Vista or Office 2007). It's just Macs where there is generally a problem. As Oli pointed out, SS Gold has radio formats.
On the Mac side, Montage from Mariner Software is shaping up well.
Although technically not a screenwriting app, Scrivener from Keith Blount is a fantastic application for constructing and drafting documents, and includes templates for Screen and Stage plays. John Rogers gave it a great write up, and I believe it's being put to use creating the new season of Spooks.
The trouble with saying "Don't put in information which an actor can't perform in writing the script" is where do you draw the line? Every actor and director interprets a script differently and any writer is bound to come up with something that is hard to understand.
Paul,
The best way to get a proper understanding of these issues is to read scripts, read scripts and read scripts again. It's about becoming familiar and comfortable with the 'language' of a script, which everyone on both sides of the camera understand.
M