Yes, motor memory does count for a lot in spelling. I know that not many people write much by hand these days but, if you do, you will notice that many words which you have frequently written don't require much conscious thought at all (apart from thinking the word in the first place); you think the word, your hand writes it. This is the power of muscle memory. I am sure that touch typists develop much the same muscle memory for words that they frequently type.
But handwriting has to come first because the very best way to learn a spelling is to break a word down into its very smallest sounds (phonemes) and then spell each sound in the order in which it comes in the word.
So, for 'easy' words with simple spellings, such as 'caravan' you would break it into its sounds /k/ /a/ /r/ /a/ /v/ /a/ /n/ and spell each one, 'c' 'a' 'r' 'a''v' 'a' 'n'. Ideally you would say each sound as you write its spelling as this helps even more to promote muscle memory of how the sound is spelled in that word. If a child is writing the word I would then ask them to sound out and blend exactly what they have written to check that they do have all the right 'sounds' in the right order.
This can often be done without ever having seen the word but for more complex words or words with alternative sound spellings it is best to let the child read the word first (by sounding out and blending if need be) and to note any bit of it which might be tricky to spell. There' usually only one 'tricky' spelling in a word, all the rest is straightforward.
So, for more complex words, such as 'accident' (mentioned upthread); Read the word, identify its sounds and note that the /s/ sound is actually spelled with a letter 'c' ('c' with 'i','e'. or 'y' after it is most likely to be said as /s/). The sounds (phonemes) are /a/ /k/ /s/ /i/ /d/ /e/ /n/ /t/. This is the sort of word where children can't remember whether or not it has a doubled letter (or where it goes..), relating the letters to the sounds they spell makes it absolutely clear that 'cc' is needed - the first 'c' spells the /k/ sound while the second spells the /s/ sound. The doubled consonants also indicate that the vowel sound is probably /a/ as in 'cat'; this is not so important in this word but it can help with sorting out whether a doubled letter is needed in a particular word. I always have problems with 'occasion' (I often spell it as ocassion first time round
) but I can keep myself right by recalling that the 'o' is an /o/ as in 'dog' sound (well, roughly) so needs the 'cc' while the 'a' is an /ae/ sound so doesn't need two consonants after it. Studying words in this way for spelling is really quite interesting and gives lots of useful pointers for spelling unfamiliar words.
Once the word has been spelled (saying each sound as it is spelled) it should always be sounded out and blended to make sure every sound is there and in the right place.
This is what I meant by relating words to their phonic structure. it is simple and far less stressful than trying to memorise a string of letters in the correct order.
A useful 'trick' for words whose pronunciation has wandered wildly away from the way they are spelled is to say them in a 'spelling voice', pronouncing every sound; Wednesday becomes 'wed nes day', 'library' becomes 'li bra ry' etc. Nmemonics can be helpful but are best kept to a bare minimum to avoid memory overload.
Sorry this is a bit long, it's easier to do than to explain! Hope it helps.