Please think: as a native speaker of English (or maybe some other languages), how often do you forget how to spell a word? I guess that situation hardly happens.
As a native Chinese speaker, I seldom forget how to write characters. If I do forget, I would look it up with phones, computers or dictionaries, or ask someone around. If I can do none of these, I tend to write a character which looks similar, and try to use that as an aid to recall. If that doesn't work either (which is very unlikely to happen), I will write Pinyin, or just find a synonym to replace the word, or try to rewrite the sentence.
When one forgets how to write it, he/she does not have totally no clue about it. Usually it happens when a character is infrequent and has seldom used parts. A common example would be 嚏 in the word 喷嚏 (sneeze). This character is only used in this word, and consist of an extremely uncommon part 疐. Thus, many people may forget how to write it.