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I've posted this guide in a few places before now, and more people seem to appreciate it than not, so I figure it might help some people out. Bear in mind, this is how I personally write spells and/or rituals. Other folk are very likely to have their own ideas and systems, and they'll probably work just as well, if not better.

1) Statement Of Intent
This is what you want to happen, the outcome you want to acheive, the thing you wish for, and so on. Think long and hard about it, then write it down on a bit of paper. For some psychologically-related reason, it seems it's better to begin it "I will" rather than "I want"; for example, "I will get a new car" rather than "I want to get a new car". As well as that, /don't/ write out your intent with negatives, for example, use "I will be healed" rather than "I will not be sick anymore". The reason for this is apparently the conscious mind has trouble not imagining things its been told not to imagine. Don't think of a glass of purple water, for example.

2) Fulfil the 5 Key Components
To me, a spell is a symbolic act that manifests a desire. As such, you have to make everything you do in your spell symbolise what it is you want to happen -- I've divvied all the possible things you could think about into five areas; Physical, mental, verbal, somatic, and symbolic. Each of these things (to me, anyway) should be represented in a spell to give it the oomph! that's needed. I've assigned each of these (rather unoriginally) to the pentagram-shape, to give you an idea of what you're going for -- bear in mind everything you do in each of these areas should be symbolically tied to your intent in some way.

[Earth] Physical
Physical is about space, area, tools, and so on. Things to think about here would be:
Placement: Is there somehere you can go to make your spell a wee bit more effective? For example, it'd be easier to summon a bunch of dryads in a natural place such as a forest.
Sound: Would it be appropriate to use music during the ritual? Maybe ambient background music would help, or Buddhist chanting?
Lighting: What kind of lighting will you use? Candlelight? Moonlight? Natural-light only, or maybe tinted lightbulbs?
Circles: Is a circle necessary or desirable? Generally speaking, you won't need a circle unless you're calling s**t up, but it's good to play it safe.
Equipment/Tools: What equipment will you need? If you're using a circle, probably the ol' Pentacle-Athame-Wand-and-Chalice shtick. What objects could work as a focus in your spell? For example, a "get a new car" spell could utilise a toy car as a focus. Maybe you could even use some sensory cues, like incense.
Clothing: Is ritual clothing needed? If not, could you colour-coordinate your clothing to tie in with the spell, for example, wearing white for a healing spell?

[Air] Mental
Mental's all about your mind - this includes astral sight and psionics.
Visualisation: Picturing your success in a spell is usually a vital ingredient; even imagining or meditating on something related to your spell will work. For example, in the "get a new car" example, you could visualise the exact kind of car you want.
Astral/Psi Sight: Will you need it? Are you going to be prancing about on the astral plane? Will you be making some sort of psi-object that you need keen psychic vision for?

[Water] Verbal
While not all spells require an incantation, they're always fun to have. Perhaps write up some sort of rhyming chant that will help you focus on the thing you're wanting.
Intonation - How do you want to sound when you say your incantation? Forceful and commanding? Meek, humble, and petitioning? Sexually alluring, husky and with a come-hither growl? How you say the words is almost as important as the words you say. Bear in mind that you may be talking to higher powers, demons, gods, or whatever, so how you present yourself could be the difference between Loki putting nails in your enemies' tires, or yours.
Incantation/Chant: Mantras are especially useful in getting your focus going; as such, chanting a simple phrase or poem will help keep your intent clear. Is there a phrase, word or poem you could continually chant?

[Fire] Somatic
Actions! Almost all spells are made up of actions of some kind. While there're no subdivisions here, that's only because there's a whole range of things you could do; from tying someone's hair around an appletree branch seven times, to burying the ashes of a bit of paper, to simply snapping your fingers.. The objects you selected in the Physical section may play a part here.

[Spirit/Quintessence] Symbolic
This is probably the most important part - whip out your favourite table of correspondances for this one. Crowley's '777' may help, but I much prefer Bill Whitcomb's 'The Magician's Companion' (Buy it now! Supercheap! 8D!). Basically, build up as many symbolic ties as you can. There're a wide range of symbols, far too many for me to subdivide this section into, but you're looking at the likes of deities, days of the week, moonphases, colour, astrological correspondance, essential oils, and so on.

3) Figure out a series of steps for the spell
If it's just a quick, on-the-fly kind of spell, everything can be done at once - holding an image in your mind, intoning a two-line incantation, concentrating on the intended target and snapping your fingers or something.

With a bigger spell, work out a set of steps to take. A good one may be: cast circle (physical), invoke gods, concentrate on image (mental), perform action (somatic) while saying incantation (verbal), banish gods, close circle. The steps for Symbolic aren't there simply because everything you're doing should be symbolic of your intent.

Et voila, ein spell. Comments, suggestions, questions and critique, plz.