Nearly all the reversing I do anymore is serial number reversing. As Wackyass said, just changing the odd jmp instruction is not very interesting. Reversing the serial number generation functions is a much more intellectual challenge.
My procedure for doing this is to first find the piece of code that performs the SN check. I then step through each function to find out what it does. Next, I write a C function that does the exact same thing. Finally, I write another C function, if possible, that does the inverse. When finished, my inverse functions are an exact key generator. The challenge is in creating the inverse functions.
As examples, the InstallShield creation programs have SN's that can be inversed in a straightforward manner, although finding the key checking section of code is a bit tricky. On the other hand, the Wise creation program is much more difficult. Here, one cannot create an inverse function. One has to fully understand the author's method in order to reverse it.

jsteed