Rituals are tools used by the magickian in order to focus the Will and accomplish a goal. Magick is, after all, the art of causing change whether internal or external through the use of the Will. My views on rituals are that they serve either of two purposes.
1. To assist the beginner to learn to shift mental gears (achieve meditative state at will), to focus his/her will, to tune the mind to a particular energy type and/or goal through a set of symbols/imagery/actions, and/or to gather energy for working in an orderly manner. Rituals can function like mental training wheels at first, but are still sometimes good to use by the advanced practitioner if additional focus is required.
2. To be used as a formality when necessary. Some spirits require certain rituals and formalities and won't respond to the mage unless they are used.
There are many books available on the subject of ritual magick today, so I won't attempt to write another one online discussing it too much. For more info on understanding ritual structure and use, check out Excerpt 25 in my personal writings area.
Here are two examples of rituals used by the Golden Dawn (complete with notes and pictures) that are very effective for banishing or invoking, and are commonly used to begin and end a ceremonial working.
Lesser Ritual of the Pentagram (Fully Expanded version) - The most well known version (Golden Dawn) complete with energy visualizations. Far more effective than performance without the visualizations ;)
Notes on the Lesser Ritual of the Pentagram Read this before performance!
Elemental Pentagrams- For use in other pentagram rituals. See Golden Dawn and related systems.
Lesser Ritual of the Hexagram (Expanded Version in the G:.D:. style) - This is probably the best version to learn at first.
Signs used in the Lesser Ritual of the Hexagram
Notes on the Lesser Ritual of the Hexagram
Supreme Invoking Ritual of the Pentagram - This is the Golden Dawn version. Makes use of Enochian names from the Tablet of Union (or Black Cross on the Great Table). Normally performed after L.R.P. and L.R.H.
Golden Dawn Elemental Grade Signs - If you aren't sure what they are or why they're used then you'll want to read this before attempting any G:.D:. Rituals.
Modified/Kabbalistic Lesser Ritual of the Pentagram - This is the version I currently practice. I've modified it to reflect traditional (non-Golden Dawn) Hebrew Kabbalistic attributions. The 3rd part (invocation of archangels) has been changed to more closely match the original Jewish bedtime prayer (which is where that part of the G.D.'s LRP was borrowed from). I find that this version is more effective for me overall.
Modified/Kabbalistic Lesser Ritual of the Pentagram for Southern Hemisphere - Use this one if you live in the Southern Hemisphere.
Notes on the Modified/Kabbalistic Lesser Ritual of the Pentagram - Read this before performance!
Here are the Aleister Crowley versions of the Lesser Rituals of the Pentagram and Hexagram. I have included them for comparison in wording and structure.
Lesser Ritual of the Pentagram (Crowley Version)
Lesser Ritual of the Hexagram (Crowley Version)
Greater Ritual of the Pentagram - Crowley's stripped down version of the S.I.R.P. without Enochian Tablet of Union names and without Kerubic and Spirit Sigils.