Short, sweet, and seemingly obvious tip, though those I've shared it with say they hadn't seen it elsewhere.
You know those bags or plastic boxes with pre-washed salad greens, e.g. baby spinach or arugula? The ones that are a tad overpriced, and tend to taste a little sterile in salads unless you take extra care?
I never use 'em on salad. I keep one of baby spinach and one of baby arugula in my fridge, and I throw a handful of one or the other into almost everything I cook. It mostly disappears...but you get the nutritional kick, and, insofar as they're palpable in the result (usually they're not, really), they seem to always work harmoniously. I add arugala when I want to inject a little bit of bitterness, e.g. when I'm worried my sauce is too sweet or two simple. Otherwise, a handful or two of baby spinach has become a knee-jerk cooking move for me.
When I want more presence, I throw several generous handfuls in. And it's never failed to integrate well, with absolutely no special handling or consideration (at least in the sort of thing I cook). I don't worry about timing at all (unless I'm cooking at high heat, in which case I add it late to preserve vitamins).
I usually have swiss chard or collards in my fridge, but they're nowhere near as versatile. Aside from the greater cleaning/prep time (and cooking time), they affect the moisture balance of the dish, and can't help but stick out. The prewashed spinach and baby arugala are awesome quiet team players.
The sole problem is the cost factor. But I'm cooking for myself much more than eating out these days, and the savings is so great that I don't shirk from using pricier ingredients...it's still cheaper in the end.