Working on the Gringo accent
I first learned Spanish pronunciation at the age of 13 but couldn't do the r or rr correctly. I gave it a good try but at that age few kids want to focus on things they can't do well. Now, after working for 30 years with monolingual English-speaking kids who don't pronounce the r of American English (among other sounds) very well, I have some insight. In American English the r following vowels requires a lot of tension (contraction of muscle) in the tongue. This muscle contraction causes the position of the tongue to be toward the back of the mouth. In Spanish on the other hand, the tongue is toward the front of the mouth, and is relaxed, not contracted. When I again began practicing Spanish pronunciation a few months ago I had several native speakers to encourage me so I tried harder with the rr and r. I spent an entire 20 minute car trip to my cousin's saying ferrocarril. Sometimes the sound would come out right, sometimes not. Sometimes I could say the word correctly two or three times in a row, sometimes not. After a week or two I estimate that I had 85% accuracy. I am so happy with this success that I make sure to practice my Spanish pronunciation every chance I get. ¡ferrocarril! 
7 Answers
Children have the wonderful opportunity to make nonsense sounds for hours in public and to either (a) be completely ignored while doing so or (b) be praised for doing so.
Adults...not so much. Your social estimation would diminish somewhat.
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Stuff about Rolling RRs
Lazarus explains how to make the RR sound.
For some reason, webdunce explains RR again.
Most important thing, it is all about the tongue's tip resisting air flow and a balancing of those two opposing forces resulting in the vibration of the tongue's tip. There is no way a person can move their tongue fast enough by way of muscular up-and-down movements. I feel most people who struggle with this sound are trying to move their tongue fast enough some kind of way, which not even the most gifted Spaniard can do.
Cheers.
In relation to the Spanish "r" I have been told by native speakers that there only exists one (r). The rr is a separate letter that (I'm assuming) English speakers have created to assist in learning the language better because we do not have strong and weak r's in English. Basically in Spanish, a singular r is a weak r, similar to the English one. The r at the beginning of a word (Rafael, roto) is a strong r as well as the double one found in desarrollo. Because these are strong r's, you just put more emphasis on these. This may lead to a rolling sound at times. I hope this helps you with your r's!
That's nice insight, Sabor, although I believe that the "Gringo accent" goes beyond the inability to pronounce the "rr".
Doubtlessly there is an element of practice involved. However, I believe there is also some natural predisposition that makes it easier for some people to pick up languages and pronunciation.
I find that I can approximate foreign pronunciations with relative ease, which enables me to concentrate on vocabulary and grammar, rather than on just how to pronounce things.
Several of my relatives, however, seem to have a very hard time adjusting their patterns to different pronunciation modes and foreign sounds, so they must struggle harder if they wish to learn a new language, or even adjust to the local accent when going to a different country that speaks the same language.
Hmm, interesting. I was wondering if I just needed to accept the fact that I cannot roll my rs. I can pronounce the r fine put not the rr. I've asked many a person if there are specific ways in which I can learn to roll rs but no one has every come up with anything helpful. Any tips other than simply try?
Go here and click on "lugar", then "coronal", then "[r]". Push play to see how your tongue is supposed to move and listen to the the examples.
Web dunce says: "Children have the wonderful opportunity to make nonsense sounds for hours in public."
Yes, that is true but while studying for my Foundation Degree in Early Years Education at University and during my Nursery nursing training previously I learned that young children, during the process of learning to speak begin to speak with a repetitive babble referred to in Psychology as 'echolalia' which does not make sense to the listener, but, in my view this experimental bahaviour with sounds may well be helping young children develop the muscle control later required for making accurate sounds. Echolalia is only a temporary stage in the development of speech and gives way to one, and later, two words sentences, such as 'daddy gone' or 'want milk!' the latter at around 18 months.
The child's rapidly developing brain my be learning and memorising the relative positions of the tongue to enable and facilitate the manufacture of new sounds for speech patterns from which true language develops This is only my theory unless research has either already proven or disproven otherwise.
Young children do enjoy playing with sounds and laughing and having fun talking nonsense at times during their play.
There are 2 different single r's, I'm told. The medial one is like a d in the name Sara, for example. The other is like r in initial and final positions of words. reir That one is like rr but only lasts half the time.
As Webdunce almost suggested, you need to be like a baby or toddler playing with sounds, totally unselfconscious, focused on tongue, lips, jaw and the feedback going in your ears.
Another thing that helped me was to pronounce an English h in front of any Rs to be rolled. I think of the tongue action as the Bernoulli effect triggered by the h. It may be all imagination, but it helped me.
