Best way to improve hand speed?

I know this probably sounds too generic to answer, but I thought I'd ask anyways.

I'm trying to improve my handspeed and I've heard of several ways to do it. Shadowboxing with light weights (2lbs - 3lbs) is one method. Using a rubber band resistance is another method used, like a pro shadow. The final method I've heard used before is just shadowboxing.

I'd like to hear about what methods you guys use to improve your hand speed or shoulder strength. I do think that the speed of your punch has some correlation as to how "loose" and relaxed your shoulders or, probably has to do something with lactic acid buildup. Suggestion are appreciated, thanks!