How to capture and handle failed INSERT

Hi,

I'm using this code to increment a counter table:

IF Exists (SELECT keyfld FROM counter_tbl WHERE key_fld = 'key_val') THEN
    UPDATE counter_tbl SET  counter_fld = counter_fld + 1
          WHERE key_fld = 'key_val';
  ELSE
    INSERT INTO counter_tbl(key_fld, counter_fld) VALUES('key_val', 1);
END IF;

Now, I assume that it's possible for another session to INSERT a row
in the microseconds that exist between the Select and Insert
statements above.

I also assume that I can wrap the above code in a transaction, and if
the transaction fails (because another session's Insert causes my
Insert to fail), then I simply need to re-execute it once. (Updates
should never fail.)

Does anyone have a simple example of the best way to code this type of
transaction- and the best way to re-execute the same code on failure?
I could use a loop but I'm not sure if that's the best solution.