I wrote a bit about it here: http://www.quora.com/Meditation/Whats-a-nice-little-cheat-sh...

Unfortunately, I don't know the traditional lore that lets someone assess and customize a concentration object for someone else. So what I wrote there comes from my understanding of NLP, specifically, the sensory modality. It is incomplete.

I will say though, find something that feels more or less right and then stick with it (at least for the beginning). Just like there is never a perfect time to meditate, and there is never a perfect posture, or a perfect cushion, there isn't a perfect concentration object. You might be dithering because of the anxiety that comes up when you subconsciously realize, you really are going to go looking into yourself. Though, from what you are saying, you've probably already experienced that.

I also suggest trying to set aside time every day for this. If you can anchor it to around the same time of day (such as, "before dinner", "before bed") it would help. A streak calendar app also helps. The commitment is to do a minimum amount, say even 5 minutes a day, and allow a "bonus time" on the weekends. Of course, if you feel comfortable sitting longer, by all means :-)

I don't like to meditate after eating more because of the full stomach. I suppose I should try that "fullness" sensation as a concentration object sometime.

As for drowsiness, a tip I picked up from Anne Wise's book, High Performance Mind: you can fill a glass of water and hold it. That tends to keep people awake. You can also try standing meditation, as that is more energetic than sitting.

If you are closing your eyes while sitting and find yourself drowsy, keep your eyes open. My Ch'en buddies tell me it is tradition to keep the eyes open. It's possible to do this without blinking. The first couple times, your eyes might start watering as the body adjusts to not blinking; then it comes back into homeostasis.

Postures ... they do matter, in the sense that you want to be able to fall asleep in them. If you can, try it with the spine vertical. If you are using a chair, sit at the edge of the chair, feet flat to the ground.

Incense helps in a lot of ways. It also acts as an environmental cue. So is having a dedicated space for the meditation.

As for the chaos of stimulation. This one is tricky. Yes, you can get into a trance by turning down the amount of stimulation. However, the practice of vipassana is being OK with things that come in and out of your space. A stimulus stimulates you because your awareness attaches to it, and comes off of the concentration object. What you do is stay with the concentration object while allowing whatever it is to pass freely through you. You want to stay awake, though in the deeper states, things start resembling lucid dreaming. Falling asleep is the opposite of mindfulness, that is, mindlessness.

There are sometimes when I simply sit there for a good half hour, my thoughts thrashing about. And then the second session following up on it, I hit the emptiness (for however long before I attach to something again), though my eyes are open, my spine straight and unsupported.

It's worth practicing being mindful even at greater stimulation since the skill translates to being mindful while off the cushion. To put it in a different way, you slip into flow state when you choose to, whatever activity you are doing. That's helpful for things like cooking. It is liberating when someone is yelling at you at work for whatever reason, and you're not pushed off your center; you might even help your coworker relax and deal with whatever issue that triggered the yelling in the first place.

Hope this helps.