The only rule of composing is that there are no rules. It's like Calvinball. As for my approach, I try to take a new perspective with each new composition. This might mean beginning with a title instead of a song structure or chord progression or melody and letting the rest follow. It might mean composing outside on some days, inside on others, sometimes standing on my head. I might choose an absurd set of instruments before I start writing, just to see what happens.
All this variety has a real purpose, and it's not just for variety in my music. I'm not particularly interested in variety in my results. I'm interested in inspiration. The phrase above, "just to see what happens," gets to the core of it. I've actually never tried standing on my head while composing yet, but when I do, it will be in pursuit of inspiration.
I think of it as if I have two minds that contribute to art: a conscious, judging, analytical, inhibited mind, and an unconscious, embracing, emotional, divinely-inspired mind. These might equate to left and right brain, but I prefer to call them front and back. The front is always on when I'm awake, while the back turns itself on and off willy-nilly. The front is where I store music theory, deadlines, distractions, instrument ranges, Sibelius chops, etc. The back is the source of all that is good, but because I can't turn it on at will, I need to prepare with the front mind, prodding the back to open up and drop me some gold nuggets. If you've ever had a great melody pop into your head, and you wrote it down so you wouldn't lose it, that was your back mind offering up a gift and your front mind accepting it. You need both to compose anything good.
My front mind is the reason I can crank out a tune in a couple hours. I wrote one today. I can make a bunch of random choices about the structure, progressions, etc., enough to form a complete piece, but without any inspiration, it's worthless. (Someone else might enjoy it, but until I do, no one else will hear it.) That's why I consciously vary my composing situations. Repetition and familiar settings don't yield much inspiration, but discomfort in a foreign environment might do the trick. Ever found yourself suddenly able to write a paper in the final hours before it's due? The pressure does wonders for creative juices.
This visualization revealed itself to me after I wrote "Defending Their Turf" last year. I started with a random sequence of chords (literally; I used a random number generator to produce the chord progression) just to see if it would work musically. I liked the first chord, then for the second chord I heard something a little different in my head, so I changed it. Then I lost a few hours to a frenzy of inspiration, changing every last pre-selected chord to what was coming out of my head. The piece that emerged is still my favorite original piece to date.