we should start by gathering features we may need:
1-Clear,big(big form fields,big buttons,big notifications.. etc) , Contrast-color progress numbered shapes(arrows, circles , squares ... etc ).
2-Highlight current position,done and remaining with different colors.
3- Ability to move forward and back easily.
4-Showing info(example input) next to each field will be good idea.Also you can make some watermarked dummy text to each field.
5-If you intending to use ajax,pay attention to performance and show friendly message like (we are baking the cake! please wait little more:) ) such stuff will joy the user and save you more time.
6- It will be awesome if a guide messages provided on every loaded..like(Welcome Tucson! Start from here.) & ( You still can use the regular browser up-loader instead of flash upload utility ).
7-Warning messages should be friendly too!! (Oh dear! you didn't fill the product name filed. )->preferably with a scroll to that field.
- A/B Testing!! a lot of it!!
below some examples :
hope that was useful :) .