Suggest me an idea about saving data of multistep form

Can anyone one suggest me an idea about saving data of multistep form, How can I proceed ? Same process like a shopping cart payment process...

Also I need to check,

1. 
   

   Client side validation

   
   


2. 
   

   Sever side validation

   
   


3. User can go from step 2 to step 3 or step 3 to step 2


4. File uploads


5. Auto save Should i save data step wise? OR Save data in session stepwise and when user clicks on "Finish" save data to db ?

Thanks in advance