For us, a few of the things that have helped (some are longer-term than others):
-getting our vitamin D to a better level---some people use high doses as treatment for illnesses, I haven't tried it but it's something to consider (high doses being, say, 50,000 IU which is about 2 days' worth of summertime/swimsuit sun exposure)
-vitamin C -- I use sodium ascorbate, it's cheaper for the doses the kids and I need when we're sick--tends to be at least 15-20 grams/day for me, and sometimes that high for my kids but they're atypical, most kids would be lower
-if you see good results cutting wheat and dairy when you're sick, much as it's a pain, have you considered cutting them for several months? quite a few people in the Allergies forum found that they got sick less often by cutting foods they're intolerant of (didn't help us, but I think it would've made other things less effective if we hadn't done it, and it had other worthwhile benefits)
-zinc, you can supplement zinc at fairly high dosages for a few days/a week at a time
-during an illness, cell salts have really helped this year, ferr phos seems to be the usual one at the start of an illness, and I think it's significantly reduced the severity of a couple illnesses this year, but there's a long thread on Cell Salts and there's a little $4 booklet mentioned that's super helpful
I haven't tried them yet, but a friend and I made up a batch of elderberry syrup a week ago, and a batch of master tonic. I think the idea of master tonic is that it's so scary to consume that you are frightened into good health.