She is well preserved. Perfectly alright and common, I would say.
Keep in mind that you should write it with a hyphen ("-" i.e. well-preserved as one word) when used as a compound adjective, such as when it is preceeding a noun. Consider the difference:
She is well preserved.
She has a well-preserved face.
That is my preference. I have seen the hyphenated version of the word been used interchangeably with the non-hyphenated version.
If you want ta play it safe you should type it as "well-preserved" as this would work in all situations whereas "well preserved" would not.