Quickest way to get off ship at ports?

We're wondering what steps we need to take in advance to be able to get off the ship as soon as possible at each port? What about when the cruise is over, what's the quickest way to be one of the first people off?

Also, when in a port, are we able to get back on the ship for lunch if we want to?

Thanks!

There is no restriction on when and how to get off the ship during a standard port day. Just be close to the gangway wait there and as soon as the security let you out you are free to go. Security will wait the announcement that the ship is clear from the local customs.
If the ship is tendering - it is another story. Every cruise line has own strategy, some are selling tickets for the tenders, some are issuing tickets for free but you have to wait to be called, some has no restrictions, but the usual rule is that the tours are goin first.
When you have to disembark there is a organization and you have to wait to be called, but if you have your own transfer then you will be one of the first to be called.
During stay in port you can come back on board as much times as you want to and yes you can come on board for the lunch.