Sorting by parent/child relationships

This may have been asked/answered a million times already, but what the
heck...

Basically, I have a table that maintains parent<-->child relationships
within itself.  The parent_id field points to the collection_id field.
A parent_id id of -1 means it's a root record (ie, no parent).  Pretty
simple.

Question is, how do I sort a query so that children follow their parent?

I came up with this, and it works, but I'm sure there's a better way:

SELECT *, CASE WHEN parent_id = -1 THEN collection_id||'' WHEN parent_id
!= -1 THEN parent_id||collection_id END as z FROM collection order by z;

Any advice will be greatly appreciated.

thanks!

eric