Please under no circumstances become overinvolved with the band. I have far too often had to deal with a boy/girlfriend who wants their soulmate to be the most prominent in the music (I mix bands) and to get all the attention. It is very annoying and ends up making you want to spite that person and drop them down in the mix.
That said, there are many things you can do to be helpful, go out together flyering for the gigs, help set up the tours, be involved but don't smother. Being a roadie gimp is good, the rest of the band will like if you bring the beer (if you are drinkers) and help get the cabinets and drums in. But don't then start walking over the rest of the stuff. After a show, be helpful but not overly critical, and don't go blaming others if it does sound like crap. Just be positive.
