I've been using Army Painter's Quickshade on a lot of my more recent projects, and it acts as a pretty decent gloss varnish as well as shading. It has preserved my paint jobs really well (as well as provided shading for a lazy painter such as myself)! The downside of course is if you actually are good at painting (unlike me) and don't want your models to look "dipped" it's not for you. It's also very messy to work with if you "dip", and ruins your brushes if you paint it on. It takes a long time to dry and has a habit of pooling (as it's supposed to, but if you're not careful you can ruin your models) and gets really sticky.
I also have a really old spray can of Games Workshop's Gloss Varnish. It works well but you have to be careful - too little and you end up looking grainy, too much and it's blobby. I don't think they actually make it anymore, so it may be a moot point.