Being in the pastry field, I spend quite a large part of my day it seems, cutting round things into wedges. Now as sad as that may seem, it can make a large impact on the final product if all your slices come out without crumbs and all the same size. There are a few tricks to help with this. First off, figure out how many pieces you want. Some numbers are easier than others, and there are even products on the market to make the more difficult numbers a bit easier. 
for instance, when pressed into your 8″ round iced cake, will leave impressions telling you where to evenly cut for 14 pieces. But you may not have one of these nifty tools, or you may be cutting into a different number than 14, or you may be cutting something that does not easily impress, such as a pie. What do you do then?
First off, get set up. For cakes, mousses, anything soft and without 2 pastry crusts, this will require a deep container of very hot water, a clean dry towel, a clean wet towel, a sharp knife at least as long as the radius of you cake and a ruler. For pies with 2 crusts, you can use the same method, or you can omit the hot wet knife in favor of a smaller serrated knife.
Place the item on a cutting board (it helps to have a non-slip mat underneath, or a wet towel so the board will not slip as you cut. ) If you are cutting into an even number of pieces, the first thing you should you do is cut your cake in half. The best way to cut perfectly in half, is to measure the center. Take the ruler, and as best as you can without ruining your icing, place it across the widest part of the cake, hovering if necessary, mark the halfway point with the tip of your knife, or another sharp object, leaving an impression just large enough that you can see it. Rotate the cake 90º and using the center mark, place the ruler along the cake again, marking the middle again. With 2 points now, you should be able to discern a center. From this you can divide the cake in half. Hold your knife into the container of water, until it is warm to the touch. Dry it on your dry cloth, and make your cuts. The most important thing to watch for is that your knife stays at a 90º angle to the board, so you don’t get sloped pieces (this sounds easier than it is, especially with denser cakes).
Now that you cake it halved, it is easier to work with one half at a time (though admittedly not as speedy, but you can’t have it every way). Using your center mark as a guide, and always starting your cuts there, you can begin to divide into your desired number. After each cut of the knife, (not necessarily slice divided), place the knife into the hot water until it is warm again, and wipe it clean on your wet cloth, drying each time on the dry cloth. This sounds like a lot of work, and it is, but it’s the only way to prevent crumbs from getting on top of the cake. You may need to change the water in order t o keep it clean and hot. If you cake is frosted with a soft icing such as buttercream for example, you can mark where you are planning on cutting before you do and just smooth the line out if it’s wrong later, which will enable you to go back and double check your lines with a ruler first.
Using the basic technique of marking a centre with a ruler and cutting from the at point, you can quite easily divide the cake into most numbers that you will come across. The exception for this is 10. I have seem a lot of cakes divided into 10 (or 20 for buffets or rich cheesecakes).
To start to cut into 10 or 20, you must first halve your cake. The next step is to lightly using your knife, mark what one fifth slice of this half would look like (it may not be perfect, but it will probably be quite close). The more accurately you make this cut, the more even your slices will be but it’s not going to look too bad whatever happens. Once the first fifth slice is done, all you have to do is divide the remaining cake into 2 and then each slice into 2 again. Do this with the other half, and you have 10 perfect slices of cake. Half each slice again for 20.
