1. identify the deity:
2. state the reason he or she is needed: and
3. invite him or her in.
You identify the deity through attribute listing, just like you'll find the ancients did. Then you explain how you seek help in one of the areas that fall under the deity's expertise or domain. Finally, you ask the deity to be with you to provide this help. Here's a short example of how following these three steps would result in an effective invocation of Hecate:
Queen of the dark sky, be with me this night.
Cunning mistress of dark magicks, be with me this night.
I seek your strength in banishing all power that
(insert bad influence name) has over me.
Grant me time to bask in your caress.
Be with me this night.