Being consistent, day after day

Take sports again. If you train for a day and then slack off for a week, you won’t make much progress. In fact, you may end up straining a muscle or hurting your back because you haven’t conditioned your body gradually, as most fitness gurus recommend.
When you practice meditation, you’re developing certain mental and emotional muscles like concentration, mindfulness (ongoing attention to whatever is arising, moment to moment), and receptive awareness. Here, too, consistency is the key — you need to keep it up and keep it regular, no matter how you’re feeling from day to day. In fact, your feelings provide the fodder for your meditation practice, as you expand your awareness from your breath to include the full range of your experience. There’s no special way you need to be —just show up and be yourself!
As one old Chinese Zen master used to say, “Sun-faced Buddha, moon-faced Buddha” — by which he meant, happy or sad, energetic or tired, just sit as the being you happen to be.
Be especially wary of two extremes: laziness or self-indulgence (“I’d rather be sleeping, resting, watching TV”) and perfectionism (“I’m not ready to meditate. I’m not smart or good or focused enough.”) Remember, I’m talking about meditation for beginners here — and besides, the best way to become “good enough” to meditate is to just do it!