You may have actioned some standard exports from the system e.g. export contacts, export a group...but you realise that you would like more detail to export, in particular your custom fields...
A: Run custom reports within which you can include your custom fields and to select certain options from within those custom fields.
Hints:
1) ALWAYS wait until you receive the confirmation email that your report has completed before you open the report to export it to a .csv file (e.g. Excel)
2) Try not to overload each custom report with too many fields and/or too many options. Better to run a few reports successfully rather 1 overloaded report struggling.
3) Once a report has completed you have the option within the listed report to "Create as Group". Remember that the group you create is not dynamic so will not auto-update with new contacts added etc.
4) If you are planning to run the same report on a regular basis e.g. monthly, you do not need to re-create the report but just use the "Refresh Report" link and it will re-run against all your current contacts.