1 - Panels/Page Manager
If you're using Panels and Page Manager (part of Chaos Tools) then you can break up the
node/add form by enabling/overriding the node_edit page manager handler and creating a Panels layout to show the individual fields.
Just set a variant to only react on creation of your particular content type, and then arrange the form fields as you wish. This will let you split out any form fields, as well as provide a General form pane that will add any fields that you've missed out.
IMPORTANT NOTE COURTESY OF LETHARION
Something I didn't realise due to the fact that I always put the general form on the page, is that if you DON'T output ALL fields, then "form api will interpret it as "the user deleted the content of those fields" and will send back empty values. This could result in the loss of existing data, so be careful.
2 - Solely Fields/Field contrib
You could also try Field Permissions if the form is permissions based, or use a Field group with some collapsed fieldsets or tabs etc to initially hide the fields if they are not permission based.