It's hard to remember its spelling because it has so many syllables, I often say it as 'on-na-ma-to-pee-a'... and even if I pronounce it correctly, the last few letters are a nightmare for me. Is there a mnemonic for 'onomatopoeia'?
I agree with Spehro Pefhany that Yoko Ono is the place to start. When she wants to do yoga, what does she use? An Ono mat. For the poeia part, just use the vowel order of the word poetical. And what do these two pieces of mnemonic have to do with each other? Zero—that's the o that links them: onomat-o-poe[t]i[c]a[l].
The simplest way would be to learn the Greek word elements. Greek onoma (word, name) and Greek poiein (to make, to compose). The part -noma in the Greek word ónoma is connected with Latin nomen, French nom, German Name, English name and noun. With the Greek verb poi-ein are connected English poem, poet, poetic and poetry,
The Greek word onomato-poi-ia contains the genitive form onomatos, in the word composition the s is omitted. poi-ia is the noun from the Greek verb and means " the making". The whole word composition means the making of names/words, literally "word's making" or "word making".
The Romans took the Greek word and gave it the form onomatopoeia. They replaced the letter i by e. And that is the way it is written in English today.
If you have difficulties with the long word I would not use it. You can replace it by sound imitation or words that imitate sounds.
By the way, I consider the faculty to invent mnemonic aids a very helpful means . The more one trains such tricks the better one gets. It does not matter what one invents, the important thing is one does remember it. If you can't remember it you have to invent a better aid.