Randori - grasping order out of chaos.
DVD's, seminars? - don't buy it. The only way to improve is to do lots of it against a variety of people.
Develop what the French call sangfroid.
i don't think so. you can do 10,000 times of wrong things. what you will be good at is doing wrong things very well. there are lots of thing to consider in randori and you need to structure your training for it. first, consider the attackers. most attackers don't know how to attack in group. so you need to teach folks, ukes, how to synchronize their attacks so they can come at different angles and different levels, at the same time or close to it, so they don't let up the pressure on the nage. they need to learn strategies on how to corner nage. on the nage side, you need to understand spacing and timing in a group dynamics, which is quite a bit different than one on one. there are things you should do and shouldn't do. there are tactics and strategies to employ. folks tend to not know what they don't know until they got dog piled.