We've battled with the after effects of flu all Swampy's life (he had it twice as a kitten, which is why he wasn't wanted, except by us). For a few years we had good success with Interferon Alpha drops in his eyes (interferon being an immune system booster), but then just fucithalmic helped. The eyes are linked to the nose in cats, which I'd not known before, so it could have a slightly helpful effect. When Swampy is snotty and making globbing noises when he swallows, we always treat his eyes and think it helps a bit. I had a brilliant vet in Cape Town who said not to bother with the interferon injections - they don't really work and just empty your wallet.
Not found anything that actually really boosts the whole immune system though, other than keeping the purr count up (stress is bad obviously). Keeping the air in the house or flat not too dry and not too moist and also giving inhalations was what we always did before. What someone suggested was those electric air diffusers that people use when young children suffer from croup (sp) - humidies the air, if you have the central heating on.