Developing focus of any kind is a skill like any other, and it requires practice. Fortunately, you don't need only MA practice to develop focus. Variety is nice. If by focus you mean concentrating on getting things done perfectly or bothering to do them at all, then there are some things you can do that certainly aren't going to hurt.
1) Read a lot. Reading makes you smarter, which, like being stronger, is always a good idea. I'm not talking about reading facebook updates, but sitting down with a good book, preferably written by a good writer. I didn't realize how beautiful language could be until I read Charlotte Bronte. Be careful or you might learn something.
2) Learn another art form: music is probably the most potent of them all in terms of cognitive development. For example, learning how to lock up with the drummer requires a timing perception far finer than any martial artist, or learning how to play a fretless instrument in tune requires hearing a pitch in your mind's ear, analyzing the pitch of what you're playing, and correcting it before anyone else can hear the difference. This is all in milliseconds. Music is good because you have to do everything in real time, unlike say painting where you can go back and fix something. Once you've played a wrong note live it cannot be fixed, much like you can't un-take a punch. You can only give one back.

Music can also be frustrating beyond what most people experience, and learning to deal with the fact that you suck at this song or scale right now carries over into keeping focused and calm when trying to learn MA techniques you find challenging. I think the only people who could understand the frustration musicians experience are those who play Dark Souls.
3) Meditation. There are a gazillion studies on the benefits of this practice. I don't meditate. Perhaps I should meditate on why I don't.
4) Physical conditioning. I like running, because I hate running. When you're running and reach that wall, learning to push through will help you in other frustrating endeavours, like you know, martial arts. Anything gruelling and unpleasant will do.
