performance tuning queries

Hi All;

I'm looking for tips / ideas per performance tuning some specific queries.
These are generally large tables on a highly active OLTP system
(100,000 - 200,000 plus queries per day)

First off, any thoughts per tuning inserts into large tables. I have a large
table with an insert like this:

insert into public.bigtab1 (text_col1, text_col2, id) values ...

QUERY PLAN
------------------------------------------
Result (cost=0.00..0.01 rows=1 width=0)
(1 row)

The query cost is low but this is one of the slowest statements per pgfouine

Next we have a select count(*) that also one of the top offenders:

select count(*) from public.tab3 where user_id=31
and state='A'
and amount>0;

QUERY PLAN
-----------------------------------------------------------------------------------------------------
Aggregate (cost=3836.53..3836.54 rows=1 width=0)
-> Index Scan using order_user_indx ontab3 user_id (cost=0.00..3834.29
rows=897 width=0)
Index Cond: (idx_user_id = 31406948::numeric)
Filter: ((state = 'A'::bpchar) AND (amount > 0::numeric))
(4 rows)

We have an index on the user_id but not on the state or amount,

add index to amount ?

Thoughts ?