The cruise lines tend to upgrade people who have sailed with them before, or people who booked with a high volume travel agent, or people celebrating special occassions (honeymoon or anniversary), or just random people.
I would think that they would upgrade people if the rooms are empty, but you never know. I have seen that they try to sell the upgrades at the pursars office if there are empty cabins.
If you have seen the price of your cruise drop dramatically since you booked then call your travel agent and ask if he/she can get you an upgrade or a cabin credit. I did this yesterday and got a one catagory upgrade on my sailing in December.
I doubt it. Uniformed staff are, on most cruise lines, absolutely prohibited from accepting tips. It's not like a New York restaurant. Tips in the dining room, on the other hand, can be very effective.
The common wisdom is to book the highest level cabin you'll be happy with and not count on an upgrade.