All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
First, the verse that is mentioned in the question (which means): {Indeed in the Messenger of Allaah (Muhammad ) you have a good example to follow.}[Qur'an 33:21], mentions the expression "good example", i.e. someone whom people imitate and follow. This Qur'anic verse is a great principle in following the example of the Prophet in his words and action. However, the verse does not specify in details the stages of this imitation and following it does not specify what is an obligation for us to imitate and what is only desirable. However, there are details about this in the books of jurisprudence.
Imitating and following the Prophet is not restricted to the principles only, but also in branches. For instance, washing our private parts after relieving oneself with our right hand is forbidden according to some scholars as the Prophet forbade us from doing so, and this is included in the branches. Washing our private parts from urine and excrement is an obligation and this is also included in branches. Eating with the right hand and the like are also among the matters in which we imitate the Prophet .
However, one should know that there is a difference between the Sunnan (plural of Sunnah) referring to actions of worship that we imitate the Prophet either as an obligation or just as something desirable, and the Sunnan referring to habitual acts that the Prophet used to do like combing his hair, (the manner of) turning back, having a bath from a container, the way he used to walk, and so forth.
A Muslim is not obliged to imitate him in these things although following his example in such matters means following the best and complete example, but whoever does not imitate him in such matters, is not sinful, and it is hoped that whoever imitates him, will be rewarded for that.
We hope that by now we have clarified some important issues to the questioner, which is that imitating the Prophet could be an obligation, or something desirable, either in principles or in branches and that there is a difference between the Sunnah about actions of worship and a habitual Sunnah.
Allaah knows best.