there are a few tips in here Nathan.
http://www.mikesalway.com.au/nightsc...phy-101-intro/
a faster lens is a great advantage as you can capture more signal. stacking also helps although may be difficult with the setting you've got here with the tower (you end up with a composite image).
I suspect some processing will be able to squeeze some more out of this image. also note that dslr's are much noisier in warmer weather especially vs winter, so perhaps looking at ways to cool your camera before shooting could bring some benefit.
cheers
rusty