With /w/, you likely notice students make mistakes with only certain words, right? Since Japanese has /w/ in words like "watashi" and "wakaranai", or loan words like "week" (ゴールデンウィーク), it might be a matter of a cross-linguistic interference (i.e. they don't realize the "w" sound in Japanese is not the same as the /w/ in English).
With pronouncing [w], I would try to make them aware of what they already know how to do (pronounce [w] at the beginning of words like "water," "why," and "week") and use it as a springboard to what they don't know.
To make matters more complicated, though, you must get them to realize that the /w/ in English is always rounded. They're probably messing it up because in Japanese, the sound they think is like the English "w" only sounds like it (in Japanese, they don't round their lips when pronouncing "watashi" or "wakaranai"). That's the trouble with using your first language to understand a second: They think, "Oh yeah, the 'w' sound--like in the word 'watashi,'" but the problem is that sound isn't the same as the English "w". They then carry over those assumptions into other English words, and while you need not round the w in words like "water", "why", and "which," (i.e. English speakers will probably still understand you), you cannot avoid rounding in words that start with a long and short "oo" sound (would, woo, woman). Not rounding makes them sound like "ould," "oo" and "ooman."
Does that make sense?
It's the same problem as an American thinking the "r" in Japanese is the same as the "r" in English. They are clearly totally different (in Japanese, it's a flap, and in English, it's an approximant), which is why an American who doesn't understand this and tries to say a word like "taberareru" will sound like Scooby Doo.
I hope that makes sense and you can teach these sounds more effectively.
Have a good one, and good luck out there!