"in an improper manner"

That is the most tongue slipping term I have ever met and I pretty much conquer it after practicing dozens of times. 

The long "e" sound in the word "these."   Why? I don't know.  My students tend to pronounce it like "this."

The expression "where were we?" kills them. 

squinting squat anxiously reassuring urgent error failure inconveniencing  nurturing adequately sighed

intrigued apprehensive world  wonder  wander usages disposition  murmured overwhelming ritual

I teach the three sounds for -ed   at the ends of regular past tense verbs, but sometimes the student doesn't know how to pronounce the present tense of the verb.  Therefore, they are lost when trying to figure out how to pronounce the  "-ed."

long i: vs short i

l in life and n in knife

Many other sounds that are difficult for me to pronounce.

But I just try to say, say, and say again.

 First of all, the 'TH' sound, in words suchs as thing, think, the, thermal and so on and so forth. That is to say, the difference that lays between /ð/ and /θ/. Then, you can underline the different pronunciation of 'the'. For instance, the old vs the boy. Secondly, the different sound in words like ship and sheep [ʃɪp] [ʃiːp]

If I come up with something else, I'll let you know.

"Suggestions", He is "suggesting", and of course "Massachusetts".

A lot of English learners pronounce both "of" and "off" as [of].

"Admiration" and "inspiration" not difficult but confusing as in the base verb the "i" is pronounced like a "y".

Those are the ones I can think off the top of my head.