General synonyms:
* Agile
* Flexible
* Supple
* Graceful
* Nimble
* Slender
* Lean
* Elastic
More specific synonyms:
* For describing a person's body:
* Svelte
* Willowy
* Athletic
* For describing movement:
* Fluid
* Smooth
* Effortless
* For describing something that is easily bent:
* Pliable
* Yielding
The best synonym will depend on the specific context in which you are using it.