Optimizing query

I have a problem creating a usable index for the following simple query:
SELECT * FROM my.table WHERE node = '10' ORDER BY id DESC LIMIT 1

id is a serial, so the query is to find the latest entry to a given node
and id is the primary key.
The table contains around 1 million records and the query takes around 2
seconds.

I have tried to make an index on node and also on both id & node, but is
doesn't lower the
query time.

What am I doing wrong ?

Thanks,
Poul