Since an American explained how to pronounce a
French "R" (and she did it well), I, as a Frenchman, feel entitled
to write how to pronounce an English "R". Indeed, this is one of the
many difficulties in English pronunciation for foreigners. For example, ponder mentions that the Chinese have great difficulty distinguishing the consonants 'R', 'L' and 'W'. Hence jokes about flied lice. Don't underestimate the consequences of mispronouncing an English "R" in a restaurant!
To pronounce the "R" sound correctly, you should practice the
following exercise:
- lift the back of your tongue in the direction of the palate,
- but stop before you reach it!
- now you are ready to say "R".
The tricky part is to decide where to stop in step 2. Depending
on how far your tongue stays from the roof the mouth, your "R" may sound
too harsh, like a badly pronounced French "R", or, on the contrary,
it may be completely mute.
The difficulty also depends on the position of the letter in the
word. The worst case is when the "R" occurs at the beginning of a
word, or between two vowels. For example, I will never use the word
"roaring" because I know that my struggle to utter a sound
in this word would be pathetic.
As a comparison, the tongue almost reaches the palate in French
"R". In Spanish and many other languages, you roll the "R", a
technique that may frighten the newcomers but can actually be mastered
in minutes.
Some English-speaking people also roll their "R"s. Unfortunately,
this variant appears to be used in some parts of Great Britain only
nowadays. You don't hear it on the BBC or in American movies, so I
suppose that the people who speak like that sound old-fashioned.
(For a more accurate description, see Gritchka's writeup below, or webster pronunciation guidelines.)