Here's a secret we have learned: conversion rates drop drastically when merchants cross-sell more than one or two items. Consumers who are offered an over-abundance of information tend to block it out. It's true. We've tested it, and have found that to avoid leaving money on the table, you're better off making decisions about what would be of most interest to your customer. Have a point of view about what will sell, then prove yourself wrong or right.
In other words, you have to take risks. No, you don't have to scale cliffs or jump out of an airplane. But you do have to make some decisions. You may think, for example, that if someone is interested in a set of mixing bowls, you can safely suggest a nonstick cake pan, a muffin tin, a cake decorating kit, and stainless steel measuring cups -- in other words, a variety of complementary products from the same category.
But narrow the field to the cake pan and measuring cups, skip the decorating kit and muffin tins, and conversions on the suggested items will probably improve. Then, you can test different variations on the four to find the very best combination.
Two other thoughts on cross-selling:
Cross-sell what sells rather than what doesn't. You might be tempted to plug that rolling cookie cutter (it seemed like such a good idea last Christmas) in the hopes of reducing inventory. Truth is, if it hasn't sold, it probably won't. It's easier to increase sales on a popular item than on a dud.
Watch for trends. If a cross-sell works well for three or four days, then abruptly begins to bomb, look into it. It might be out of stock, or maybe someone posted a negative review of it, or a competitor has come out with a newer, cheaper product.