Like Dan I have altered the checkout process as much as possible, and implemented about every mod in these forums. I have spent many hours trying to make it as Б─≤idiotБ─≥ proof as possible. Last week I uploaded the new checkout to my site and my orders this weekend were up about 300%. I donБ─≥t know if it is the checkout process, or maybe just a natural growth of my site, but IБ─≥m pretty sure it was the checkout.
I did a Google search for the top ecommerce sites, and tried to make the checkout as close to the Б─≤big boysБ─≥ as possible, while still working within x-cart. I know that this has been beat to death, but I think the biggest problem with the x-cart checkout is too cluttered. I like BoomerБ─≥s CSS template, because it takes a lot of the fluff out of the checkout (pictures, descriptions, multiple invoices, login, etc)
Overstock has one page, name cc and done, some sites have multiple pages but everything is labeled and clear as possible. Half the time I was lost on my own site with the x-cart default checkout.
My favorite error message:
Б─°Your form is filled with errors!Б─²
Just a few things I did that I think helped the checkout.
Removed everything but the header
Removed, pictures, description and compacted the cart.php page down.
Login or Register on the same page
Edit the cart so you only need to enter your name once.
Put graphics on the select payment method page (every other site I looked at had this).
Moved the credit card info to the top of the page with the submit button.
Basically, so it is click, click, click, done.
Just my opinion, but it seemed to work pretty well.