Ben in Montreal is having trouble getting wet in the shower.
Often water pressure will decrease in a system when you have old pipes that are slowly collecting calcium and other minerals on the inside, closing down the diameter of the water flow. In a serious case, simply putting in standard new piping will make a world of difference.
If you already have good 1/2inch copper pipes and the flow works everywhere except that upstairs bathroom, many plumbers will recommend installing a special 3/4" line to that bathroom. This basically gives this room priority over the rest of the house. The primary draw back to this technique is that a 3/4 inch pipe holds more than twice as much water than a 1/2 inch pipe -- hence requiring much more hot water out of the hot water tank just to get to the shower, and then much more water in the pipeline that will eventually cool down wasting energy. Because of this, some people run 3/4 inch cold water and 1/2 inch hot water, getting the pressure from the cold side and tempering it from the hot side.
But if the flow all over the house is bad, then you have to look at entrance piping or even city water pressure. Unless of course, you discover that the kids turned the main water valve down without letting you know. It should be all the way open, all the time.