Related products is a killer when it comes to up-selling your products and should never be ignored. Think of the spotty teenager behind the desk when you buy a Mars bar: "As you bought this Sir would you like to look at our range of Twix's?"
When the customer clicks "Add to cart" they are immediately shown your choice of perhaps similar of maybe totally different products as something "they might like as well".
It is simple to setup although if you have a big store it is going to take a long time so reserve your afternoon (s) get a cuppa and lets get down to it.
1. Go to your first product in Virtuemart/Products in the back end of your shop.
2. I suggest you do a Category at a time so you can keep track of each product and not miss any, so select your first category from the Filter at the top.
3. Click the first Product name and then from the product menu bar select the top right "Custom Fields"
4. The second section down is the Related Products. and should look like this:
5. Type into the empty field a part name of the products you want to find to "relate" to your primary product. You will notice that as soon as you start to type the images of the products with these letters in their names pops up.
In this example i have typed the letters KIT and 5 other products with KIT in their title have popped up.
All you have to do is click one of the images and that product will become related to the primary product. Unfortunately to add each one you have to remove the text and type it again for each new one, hopefully one day this will be updated so you can immediately multi select the products you want and not have to type it in each time. I have requested this as a feature but wont be holding my breath!
It is a good idea to check your original product to see how many "related products" look the best in your setup, this can usually be adjusted slightly to look its best, just ask!
When you have enough done don't forget to click SAVE and check it all looks good before clicking "save and close"
Now plow your way through all your individual products, it is worth remembering that if a product is unpublished or removed or out of stock then it wont show up anymore on other products as "related".