Improve Query

So Google hasn't been helpful and I'm not entirely sure what  to look
for in the mailing lists to find the answer to my problem, so here
goes.

I have a query and I have run
explain analyze
select count(*)
from score
where leaderboardid=35 and score <= 6841 and active

The result is
"Aggregate  (cost=2491.06..2491.07 rows=1 width=0) (actual
time=38.878..38.878 rows=1 loops=1)"
"  ->  Seq Scan on score  (cost=0.00..2391.17 rows=39954 width=0)
(actual time=0.012..30.760 rows=38571 loops=1)"
"        Filter: (active AND (score <= 6841) AND (leaderboardid = 35))"
"Total runtime: 38.937 ms"

I have an index on score, I have an index on score and leaderboard and
active.  I can't seem to figure out how to create an index that will
turn that "Seq Scan" into an index scan. The biggest problem is that
the query degrades very quickly with a lot more rows and I will be
getting A LOT MORE rows.  What can I do to improve the performance of
this query?





Thank you so much,
ZC