Many people ask this question, and many of us members here give mixed answers. I have couple links I will be listing below that I think you should check out.
It is important that you study the structure of other spells and how they are written, and observe the image of intent that the words paint in your mind. When writing a spell, make sure to paint a clear image of intent - that is something a lot of people who write spells fail to do (and in most cases is why their spell flops).
Anyways, here are the links:
A lot of spell guides neglect one thing - and it may not seem to be that imporant, however, I consider it to be just as important as any other part of the spell/ritual - they neglect the ending. The "clean-up" if you will. Just keep this in mind when writing a spell: "What do I do afterwards?".