I've always felt that my English "r" (International Phonetic Alphabet: [ɹ]) has sounded a little off, and I've been rather self-conscious of pronouncing words such as "girlfriend," "weird," and especially "rule." One of my favorite classes I took at the University of Florida was Spanish Phonetics. We practiced lots of tongue-twisters to improve our accent, and I found them tremendously helpful. So, I've invented my own tongue-twister to help me correct my [ɹ] problem:
"Rory Roland's really weird girlfriend Aurora Rudin regularly re-routes rural jurors to Rutgers' rear recreation room where rules and regulations regarding rural breweries are rarely re-written."
Or, in IPA, /ˈrɔː.ri.ˈrəʊ.lɪnz.ˈrɪ.li.ˈwɪərd.ˈgɜːrl.frend.ˌə.ˈrɔː.rə.ˈruː.dɪn.ˈre.gjə.lər.li.ri.ˈraʊts.ˈrɜːrl.ˈdʒɜːr.ərz.təʊ.ˈrʌt.gərz.ˌrɪər.re.kri.ˈeɪ.ʃən.ˈruːm.weər.ˈruːlz.ən.ˌre.gjə.ˈleɪ.ʃənz.ˌrə.ˈgaːr.dɪŋ.ˈrɜːrl.ˈbruː.əɹ.ˌriz.aːr.ˈreər.li.ri.ˈrɪ.tən/
When I'm alone (for example, in the car), I recite this sentence several times, until my mouth is actually a little sore. I usually end up laughing to myself. It's been helping a lot, and I'm feeling more confident with my "R"s!