There is a history of presenting verbal incantations as rhyming verse. Perhaps the most famous example is the witches's curse from Shakespeare's play Macbeth. A quick review of the spells published on the internet and it is apparent that a large portion of them are rhyming. This overwhelming amount of poetic verse in spell craft can lead one to the conclusion that spells must be formulated with rhyming verse for the incantation, but is it necessary?
The short answer to this question is no. As long as your incantation assists you in presenting proper focus and direction for your magical efforts, rhymes are not needed. The tradition of using rhyming verse for spell craft is of an uncertain origin. When we consider the historic evidence of past witchcraft practices, we find that a vast majority of it comes from people of the lower social classes. (Witchcraft has historically been a tool through which the oppressed strike back at their oppressors. As such, it has a very strong association with the lower classes, which will generally include the oppressed population of a culture.) This association with lower socio-economic status often comes with a lower level of formal education.
One may ask what this has to do with the presentation of spell craft in rhyming verse. Generally, complex formulaic verbal incantations come with a higher degree of literacy and education. While rhyming is found in the spell craft of the lower classes, it does not have as great of an emphasis as that of people in higher classes, with allowances made for cultural emphasis of course.
In western magical practices, we find there are two purposes for rhyming. One is that it serves to assist the practitioner in maintaining focus upon their magical efforts and target. The other is that it is a functional component of the spell craft. Incantation and enchantment both have their roots in the Latin term incantare, which translates to: to chant [a magical spell] upon. The native English term that is corresponding to this is galdr which translates to: song or spell. Generally, however, incantation is not a necessary component of witchcraft.
While song/verse based magic does exist, it is its own class of magical exercise. In ancient times, there was a distinction made between song based magical practices and that which is based in other practices. In the native English terms, galdr was the song based magical practices. Other practices were known as seith. It is reasonable to believe that similar distinctions have been made between these two different ritual focuses, though there may not be specialized terms for them.
An additional wrinkle to the practices that could fall under the category of galdr is the fact that not all poetry or music is composed of rhyming verse. It can also include what is known as blank verse or non-rhyming poetry. Thus, incantations can be of a greater variety of forms as long as there is some sort of rhythmic stress pattern through out the charm, though this is not a hard and fast rule.