Before you start cutting and shaping foam, you should have a basic understanding of pitch and cradle, how these affect the comfort of your seat. This will also help you understand why your current seat doesn't work so well.
Pitch is the front-to-back angle of the seat. It determines how much the seat causes the rider to slide into the tank, how much pressure is placed on the wrists, and how much soft tissue in the buttocks is pushed away from below the sitting bones (ischium). A more forward pitching seat pushes the rider into the tank, places more pressure on the wrists, and pushes soft tissue out from under the ischium, thereby causing them to be irritated more quickly. The pictures to the right and below show this relationship.
The picture of the level glass shows the angle of the seat when placed on a stock Honda VFR with its cover removed. You can clearly see the forward pitch by looking at the position of the bubble. Along with other characteristics of the stock seat, this forward pitch made this stock seat very uncomfortable for me.
Below are a couple more pictures of a Honda VFR seat. The left seat has been modified to significantly decrease its forward pitch. This modification also provides a bit more leg room, given the slight increase in seat height. This can make longer rides easier on the knees. This exact modification may not work for a much shorter rider who needs to be lower to reach the ground. Also, notice that the original cover has been reused, saving time and money. It may not look as beautiful as an expensive, custom upholstered seat, but: 1) it's not that ugly, 2) nobody sees it when I'm riding, 3) it has allowed me to increase my comfortable riding time from about 30 minutes to about 10 hours, and 4) it costs me less than $50 in materials.
And here are a couple of pictures of a Suzuki GSXR600. The left one has been modified to decrease its forward pitch, and to provide more cushion and better cradling. The stock seat doesn't look like it pushes the rider forward much, but with lower clip-ons and higher foot pegs, I found myself constantly having to push myself back off the tank. In addition, the stock seat's poor cushion and cradling caused my butt to hurt as soon as 30 minutes into a ride. With a stock seat like this, it was hard for me to enjoy the bike and ride the heck out of it like I wanted to.
One part of the seat that helps decrease the obnoxious forward slide of so many stock seats is a well-shaped "nose." The pictures below show a Suzuki Haybusa stock seat (left) and a <a href="http://www.corbin.com" target="_blank">Corbin</a> Hayabusa replacement seat with a more pronounced nose. The Corbin nose not only prevents the rider from sliding forward as much, but also helps distribute weight across the seat, thereby removing some pressure from the sitting bones.
Cradling is the property of a seat that determines how uniformly a seat distributes body weight across the seating area. By distributing body weight across more seat area, pain is reduced in places where the buttocks bears a lot of weight. This especially occurs right on the sitting bones. Cradling is is a key aspect of making your seat more comfortable, especially for longer rides. The pictures below illustrate how cradling helps distribute weight to make a seat more comfortable.
The pictures of the stock and Corbin Hayabusa seats above clearly show the difference in cradle between a stock seat and more comfortable, modified seat. The pictures below show an example of the stock and modified Suzuki GSXR600 seat. The modified seat is way more comfortable, and still allows the rider to move around easily on the seat when riding the twisties.
Some seat makers, such as Russell and Bill Mayer, build custom saddles particularly comfy for cruisers and sport-touring bikes. These seats are great for serious, long-haul riding. The shape of their seats may not work for sportbike riders who need to move around a lot on the seat (side-to-side and front-to-back). However, the shape of their seats illustrates how these seat makers, based on years of experience and rider feedback, incorporate comfortable pitch and cradling.