Insert without duplicates

Hi -

This has been covered elsewhere, but the typical answers seem to
involve using triggers, etc.

What's the best way to insert a row into a table providing it's not
already there?  In my client (Python) program, I can do two separate
interactions with the server, the first a query:

   select 1 from foo where x = 1 and y = 2;

and the second the actual insert, if the query returns nothing:

   insert into foo (x, y) values (1, 2);

Or I can use EXCEPT:

   insert into foo (x, y)
     select 1, 2
       except
     select x, y from foo where x = 1 and y = 2;

Are there other variants?  What's the "best" method (fastest, etc.).
The query planner will use the same plan in the second case as the
first, no?  What if I had a handful of new rows I'd like to
(conditionally) insert - can I do them all in one statement somehow?
Remember, I don't want to use triggers or anything like that, just
standard SQL statements.

Thanks.

- John D. Burger
   MITRE