Ok so you have a site that you think is working well, but you want to make some changes that you think will improve it even more. When you make changes, what testing do you do? Have you ever run two websites side by side, basically mirrored, but with your changes on one and not the other and somehow splitting the traffic between them? So that way, you can see if the changes improve sales, or make things worse?
Yeah, I sure wish there was a way to do that! That would have saved me so many headaches over the years. Just always make sure you have everything backed up so you can revert to the original at any time if things go wrong. I'm such a nervous updater, I make three different copies of my site before I start and stick them in three different places just in case.
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Actually, I do split testing on ecommerce sites all the time. It's a great way to find out if the positioning of something yields more clicks.
It involves running your DNS through another place and then they split the traffic. It assigns a cookie to the visitor so that the visitor always gets the same looking site, but it splits up the number of visitors so you can do some A/B Testing.
I use a program that's part of StomperNet (a comunity of SEO people with SEO Tools), however Google offers the ability to do A/B Split Testing also (Google Website Optimizer)
If you're not testing, you're not moving forward. You should always be doing A/B Testing and minor improvements with everything from Adwords to page layout, colors and content. Changing from one look to another and HOPING that things are ok can spell bad news for the future.