I've been meditating daily with ony a few slip ups for a couple of months. I have a meditation group to keep me motivated (Eastrose on Tuesday nights). We do Easwaran's passage meditation. Easwaran gives a pretty indepth discussion of how we avoid going deep into meditation and allow distractions. There are lots of ways.
My typical distraction is to fall asleep. The passage I am meditating on often appears in my mind as it does on the page. As I go through the passage the words light up one at a time. When I start to fall asleep the lines reassemble in different order. They dangle like melted clocks in a surealist painting or end abruptly. These are all signals that its time for me to breath deeper and sit up straighter before I start to snore.
But yesterday morning the mind did something charming to distract me. A Hummingbird swooped right in through my passage. It hovered, as hummingbirds will, in the center of my mind allowing me to admire its brilliant blue and green colors. And then, when I regretfully realized that it was a distraction, it darted away.