WHat do you mean you've been "adding table sugar and less and less?" Are you talking about putting sugar in each bottle at bottling time? How much are you adding?
To get certain level of carbonation you need to be pretty precise in you measurement. There are calculators to help you figure it out, and it's based on the coldest temp your beer was during the process. The amount can be figured out based also on style. Though most new brewers start out with a set amount of priming sugar (usually 4.5-5 ounces) which produces around 2-2.5-volumes of co2. WHich is average for most beers.
The best way to get a consistant level of carbonation is to bulk prime, by adding the measured amount of sugar to about 2 cups of water, bringing it to a boil, letting it cool and adding it to a bottling bucket while you are transfereing the beer from fermenter or secondary, the sugar solution will mix pretty evenly with the beer at that point, and you will have pretty much even carb across the board.
If you are bottle priming, and getting sime foamy bottles, then you are probably addin too much sugar to some bottles. It's really hard to get accurate measurement in each bottle.
I go into more detail about bottling in here http://www.homebrewtalk.com/f35/bott...ebrewer-94812/
It really depends on the style and gravity of the beer in terms of not adding any sugar. With really low gravity beers, you could let it carb on it's own, in fact some beers like british mild, when you are using a carbonation calculator in brewing software, show no sugar is necessary.
But in the case of an imperial stout, which is a high grav beer, you would just end up with flat beer.