As a frequent poster on this site, I have seen many itineraries. Some are extremely well thought out, other appear to have been patched together from disparate pages of guide books. Here are some suggestions that may apply to all.
1. Don't do too much in a day. New York is a marathon and not a sprint. "I'm only here for four days and want to see everything." Well, you can't. Don't put too much into any given day. You will end up having an exhausting run-around and will have photos, but no experiences.
2. Look at a map. I've seen itineraries that bounce around from Greenwich Village, to Coney Island to Midtown and back to downtown. Plan your activities around a specific area or two for each day.
3. Leave time. It takes time to get places. It takes time to appreciate places. Most of all, it takes time to truly get the feel of a place. Seeing Union Square by walking through it is far less entertaining than hanging out for some time there - shopping at the farmers market, checking out the vendors, sitting on a bench and watching NY unfold.
4. Have an open day or afternoon. Leave time to go explore a neighborhood in Brooklyn or Queens. Leave time to just wander around Central Park. Leave time to go back to a place that you saw off of a HOHO bus. If you are with kids, leave time to have them enjoy a great playground.
5. Remember to be the master of your itinerary and not its slave. Feel free to change, rearrange, drop and add at will. New York is always changing, change along with it.
Anything you miss you will catch on the next go-around.