Re: double jab
It's tricky but once you get it it's natural and hard to explain.
Best way I can think of is to get the first jab to bounce back at you, like your arm is an automatic elastic thing.
To practice: Throw a jab, throw another jab. Try to throw each one perfectly and as fast as possible and return it as fast as possible.
Just keep doing it, sooner or later the jabs will start to flow together and get faster/start looking better.
The key is to make sure you're throwing two perfect jabs and returning them. Once you have the technique perfected you just work on throwing the two jabs faster. The time gap between them will diminish. A lot of guys get carried away practicing speed and they turn it into an elbow flick (Andre Berto style).
The double-end bag is THE tool to practice double jabs.