Many students think that grammar is the most important and difficult factor in learning English (or any language, for that matter). They are both right and wrong. On one hand, grammar is complex, separating items according to time, distance and so many other factors. On the other hand, grammar can be studied logically; it can be broken down into individual components and organized in a manner that fits our learning styles. Like vocabulary, grammar can appear to be something we can control.
Eventually, with lots of practice and hard work, grammar will become more familiar; the grammar rules will start to make sense, and even the exceptions to the grammar rules will become more familiar and seem to be more “logical” than they were at the beginning (students learning English should realize that contemporary English has had so many outside influences on its vocabulary and spelling that it is no longer a “pure” language).
However, some might argue that pronunciation is the most important and difficult factor in learning English. And, unlike vocabulary and grammar, pronunciation can't be logically organized. Pronunciation is less a cognitive skill and more like what we call “muscle memory”. In order to make the necessary sounds in English, the tongue has to assume a specific shape that it may not be used to (because it hasn't had to make that shape before). It needs to be shown how to form these shapes and then allowed to practice them until they become natural.
One tool to do this is what we call “Tongue Twisters”. The name is exactly what it sounds like: we try to twist and stretch our tongue into unusual shapes while pronouncing certain words. This might sound a little scary but it can also be fun. Tongue twisters usually incorporate one or two specific sounds and repeat them again and again. Where two sounds are spoken alternately (first one, then the other), they are often quite different and require the tongue to form distinctly different shapes with each sound. The tongue has to go back and forth between the shapes, without making an error. Tongue twisters can be really fun when a student, who has made some small improvement, attempts to make the sounds and say the entire tongue twister faster and faster.
Give this a try. There are lots of examples of tongue twisters out there on the Internet (some better than others). If you like, you can even sign up to take a trial lesson with me and I can show you even more information about why tongue twisters are unique and will probably be around for a long time to come.