I'm having trouble taking form customizations (entry and transactional) from my sandbox to a new/clean development instance. Is there something else I should be doing? At the moment I am just trying to bundle everything up and install it on the development machine, but these form customizations don't appear on the development instance. How do I do this?

If a custom field is created, and on the custom field setup the tab is specified, then the field should at least be on the right tab when you move the form in production. If however, you have a created a field to display on say the "Custom" tab of a form, and then gone into the form (entry or transactional) to move the fields around - these changes are not moved into Production its a ball ache to say the least.

I normally say "screw it" and just do my form customizations in production.

I agree, that's exactly what I do.