How can you protect your invention before you apply for a patent?

Documenting your invention. See a solicitor so that you can prove you were the first to think of it.

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Write down in detail everything about your invention including pics or drawings.  mail it to yourself certified mail (so you have to sign for it and its on record with the postal service) but dont open it.  that way, you have proof that the idea existed before anyone says they thought of it.  if you can, get it notarized or signed by a lawyer before you mail it.

No, the prize goes to the first who files a patent.  An idea is NOT patentable, only a specific embodiment of the idea."a machine that transmits sound through wires" is not patentable.  A design for a working telephone is patentable (or was, rather).

There is a well known story of another man also working on a telephone design who arrived at the patent office hours after Alexander Graham Bell.  He lost, Bell won.  Some people say the other man's device was better, but it infirnged on Bell's design, so he could not use it.  That's life.

First you need to be sure that it really is a good idea and that it is something people will want to buy.
It is also important to make sure that nobody has beaten you to it.

It is a good idea to try a thorough search of the internet first.

If nothing turns up, the next place to go is the Patent Office

By searching the records you can find out if anyone else has already patented a similar idea - even if it is being used in a completely different way.

If they have, you need to go back to the drawing board.

You can either carry out the search yourself or employ a patent agent or patent attorney to do it for you. But you need to make sure the person you employ is registered.

If the search turns up nothing you can go ahead and apply for a patent.