Everyone has their own opinion on when it is best to go. It depends on if you like long hours and everything running, or if low crowds is most important.
In my opinion, the best time to go is mid July. It can get hot, but not humid. The parks are open late. Most all shows and rides are up and running. It can be crowded, but mornings are not and with good use of Fast Passes, you keep your average wait time down to about 15-20 min. I just love the warm summer weather and long hours. That's just me.
If you go during an "off" time keep in mind that hours may be shorter and shows may not run everyday. Also, what season do you like? Would you like to see the fall decorations, the Christmas decorations, ect?
Yes, there are packages out there.
AAA and Costco have some or you can go through Disney. We always book our hotel and then buy an annual pass to last us through two summers. Before we got APs, we bought tickets through AAA to get free parking and still booked our own hotel.
As for dos and don'ts- just read through the threads on this forum to get ideas. But DO plan to parkhop everyday. There is no reason not to.
Are you thinking of going within this year? Next year?