For dust about the only way is to seal them in a storage solution that is as air-tight as possible and made of a material that won't break down fast.
Glass cabinets are popular and let you see the models clearly whilst keeping dust off them as the glass encases the model. Depending on the quality of cabinet you might find some dust seal better than others.
Another is to store them in carrycases; whilst you can't see the models you can at least keep the dust from collecting on them. KR Multicase and Battlfoam both make options for storage such as boxes just to hold the foam (as you don't need expensive cases/backpacks if the models are purely being stored at home).
If you get dust on models its not a huge problem, light dust can be blown off whilst thicker layered dust will come off easily with a light water bath and gentle rub with an old, worn tooth brush. Varnishing will help in so much as it will help reinforce the top layer on the model and reduce the chance for any paint chipping/degradation.
Another thought is if you don't have sealed shelving you can always drape a cloth over; won't stop all dust but will reduce it. Similarly there are ionizers and other machines that will remove dust from the atmosphere, another way to lower dust accumulation.
Also for varnishing tips read this
http://iwillnevergrowup.com/2014/03/28/hobby-tips-gloss-varnish-vs-matt-varnish-the-logical-myth-explained/