You might need to make relationships between a batch of your records at once. If the relationships are between the same record type, you might do your import only to find that it didn't really make the connection. Here's a method that we've found that works.
Don't forget about the contact limits for importing (depends on your server and memory available). We generally import around 200 contacts per batch. So when importing relationships, we would do no more than 100 records since relationship imports count 1 record as 2.
See our sample CiviCRM spreadsheet for typical columns used when importing.