Zara,
You are right, steroids are nasty things and you don't want to make a habit of being on them. The best way to prevent ABPA is to limit your exposure to aspergillus. One of the most common types of exposure (at least in large quantities) is house renovation. Knocking holes in walls can release huge amounts of aspergillus and was the #1 cause of hospital related aspergillus outbreaks (which is why you see them with those tents and plastic everywhere when they do any kind of work). Aspergillus can live almost anywhere, so it's going to be hard to completely avoid. Look at your home for possible areas that can host a fungus (potted plants, basements, shower drains) and try to limit time there or remove/sanitize some of the culprits.
Do you know how high your IgE is?
If ABPA flareups become more frequent some docs are treating it with Xolair, which is not a steroid. It is however new and likely expensive and its also a subcutaneous injection which turns some people off. I think the dosing schedule is 2-4 weeks.
As for me I did one (long) course of steroids with my 1st round with ABPA and I swear to god I'll never do it again. The good news is that my IgE went from nearly 400 to 47, so it was a successful treatment in that regard, but the side effects of the steroids are too great for me to consider taking them again, especially when there are other options (i.e. Xolair).
I wish you luck.