I have a male house cat (and a half, as the second is indoor/outdoor cat, he like outside potty). Anyway, before he was neutered I was starting to have problems with him going places other than his box. I discovered two issues with him... 1. getting him neutered was a very good thing and 2. don't change his litter brand, he prefers Fresh Step and if I use something else, he goes somewhere else.
I found out about the litter issue, when I ran out of Fresh Step, and my husband brought be a bag of clay oil dry stuff (looks/feels just like cat litter and many people use it for litter as it can be cheaper than litter). As soon as I used it instead of his regular cat litter, he started going anywhere but his box.
Now that he is neutered and I have a good supply of Fresh Step litter, we haven't had any problems with going outside the box.
Don't forget to be diligent about cleaning the box. A clean box is pretty important to a cat or they'll quit using the box if its too dirty.