Two basic concepts I use:

1) if you want to insert just one row:

IF EXISTS (SELECT 1 FROM targetTable t WHERE t.ID = @varId)

UPDATE

ELSE

INSERT

2) If you want to upsert (update or insert) multiple rows from another table:

UPDATE target

SET columns

FROM target t

INNER JOIN source s

ON t.id = s.id

--followed by

INSERT INTO target (column list)

SELECT source list

FROM source s

LEFT OUTER JOIN target t

ON s.id = t.id

WHERE t.id IS NULL

(Side note: don't do it the other way around -INSERT then UPDATE- since you would update the rows you just inserted...)

With SS2K8 the MERGE statement seems to make that task more efficient since you won't have to use two statements...