Instead of learning a word like onomatopeia like this:
on...
ono...
onomat...
onomato...
onomatopei...
onomatopeia
Try saying it out loud this way instead: ...a
...peia
...topeia
...matopeia
...nomatopeia
onomatopeia
This will almost always result in learning to pronounce the word much better. I say almost because there probably are words out there that violate this rule, but I haven't come across any of them yet. The reason this approach works is because the first syllable of almost all polysyllabic words in English is pronounced in a more drawn out fashion than the latter syllables. By learning from back to front, you force yourself to learn to draw out the first syllable and shorten up the last syllable(s).I have found that this rule is also applicable to almost all Romance languages for English speakers learning, French, Spanish, Portuguese, etc.
If you are a speaker of a second language, I strongly urge you to try this out yourself and get feedback from a native speaker on your pronunciation.