Push back on the manufacturers whose products generate the most returns, and encourage them to develop more standardized and obvious fit indicators. A classic example is shoes.
I've always loved Merrell shoes. They fit me really well ... at least some of them do. I didn't realize how much their footbeds varied until I tried purchasing online. Disaster. The shoes I purchased felt terrible and I had to return them. Further research revealed they make different lasts, but you can't tell online. Why can't they have some sort of indicator on the type of last — flat, more arch support, etc.
Also, help your customers find other purchasers just like themselves so that reviews are meaningful. For example:
If I live in the northeast, a winter coat review from someone in Florida isn't relevant. I want to see a review from someone in my area, or colder, like Toronto.
If I swim for exercise in my local chlorinated pool 5 times a week (which is harsh on lycra), the swimsuit review on fabric quality from someone who wore it at the beach every other day for a week, isn't relevant.
If I'm a sailor, I don't want to see a waterproof camera review from someone who got spritzed with a few drops of rain in a coastal shower, I want to see a review from the person whose kid dropped it in the bottom of a dinghy full of salt water and it didn't get noticed until hours later. Did it still work?
In short, prompt reviewers for higher quality reviews that include how they use the item, then help similar reviewers find each other.