CASE in ORDER BY clause

Hi,

I am trying the following:

critik=# select start_date from show_date order by case when start_date > CURRENT_DATE then start_date desc else start_date asc end;
ERROR: syntax error at or near "desc"
LINE 1: ...se when start_date > CURRENT_DATE then start_date desc else
...

If I remove the "desc" and "asc" then the quey is accepted but doesn't
do what I want.

OTOH if I try:

critik=# select start_date from show_date order by start_date case when start_date > CURRENT_DATE then desc else asc end;
ERROR: syntax error at or near "case"
LINE 1: ...ect start_date from show_date order by start_date case when ...

How can i order ASC or DESC depending on a condition?

Thanks,