HARDEST THING I'VE EVER DONE. Not ever, but this was pretty hard. >.<
I took a floral design class in my first semester of college. Probably the best class I've had, seeing as I've gotten to arrange a lot of plant stuff as of late. So, with a long tank, a really cool thing to do is keep a big open area in the middle with some bulky center pieces, but at the edges you'll want to gradually lift the eye-line upwards with plants. Here's an example of what I mean by that. See how it gradually lifts? You can use the same plant, or different ones.
I've found that throwing in a complementary color for rocks usually helps bring out your betta's colors more and makes them more visually appealing (as if they aren't already, right?). Don't forget your floater plants!!!