Next is the issue of heat dissipation. Now I'm not the best at math, but basically what's required is to use the Rtheta factors on the data sheet to calculate the amount of heat the package can dissipate without melting. As I've stated before, since I know I'm not the best at explaining things, here's a link (this will be a common thing, me linking you to helpful sites) that will help a great deal and walk you through the steps of calculating how much it current a FET can ACTUALLY handle.
I've found it to be very helpful, and realized that the FETs I was using could only handle about 20A before melting. Read through it a couple of times to absorb everything.