Speeding up SELECT MAX(),... GROUP BY ... ?

Hi, 

I am having select statements which are running too slow. 
It's PostgreSQL 7.2.1 and a VACUUM ANALYZE was done. 
There are various where clauses restricting the output 
to timepoints and/or ids related to sources,locations,...
One statement looks like: 

EXPLAIN 
select max(timepoint),lid,mid,sid from onfvalue 
where  timepoint <= '2002-05-01 08:00:00'  and  sid in (1,3,5,7)  group by sid,lid,mid; 

NOTICE:  QUERY PLAN:

Aggregate  (cost=21044.07..22383.53 rows=13395 width=20) ->  Group  (cost=21044.07..22048.66 rows=133946 width=20)       ->  Sort  (cost=21044.07..21044.07 rows=133946 width=20)             ->  Seq Scan on onfvalue  (cost=0.00..5530.24 rows=133946 width=20)

Any ideas/alternatives to speed this up? 
The table 'onfvalue' is supposed to keep some million rows. 

Regards, Christoph