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.
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.