Help with optional parameters

I have the need to have optional values for a query in a stored procedure
that I am building. (using postgres 8.1.4). This particular query executes
against a huge table (several million rows) and has six optional parameters
that can be sent to the function. If one of the parameters is null then the
parameter doesn't need to be included in the query. Also the values in the
columns that are optional can contain NULL values. One way that will work
(although extremely cumbersome) is to have a switch on the parameters to
execute the correct query:

--This is a sample
IF (a IS NULL AND b IS NULL) THEN
select * from my_table;
ELSEIF (a IS NOT NULL and b IS NULL) THEN
select * from my_table where a=parama;
ELSEIF (a IS NULL and b IS NOT NULL) THEN
select * from my_table where b=paramb;
ELSE
select * from my_table where a=parama AND b=paramb;
ENDIF;

This is extremely bad when you have 6 parameters giving 64 possible queries.

I tried using this (which works) but the planner likes to throw out the
index for the columns because of the OR condition:

select * from my_table WHERE (parama IS NULL OR a=parama) AND (paramb IS
NULL OR b=paramb);

My next thought was to get the planner to think that using indexes would be
a good thing so I did the following:

select * from my_table WHERE a=COALESCE(parama,a) AND b=COALESCE(paramb,b);

That works great unless the column value for a or b IS NULL in which case
NULL<>NULL because it equals NULL.

Then I used the standby: set transform_null_equals to 1

This allows select null=null to return true.

However, I ran into the problem that a=a (when a is a NULL value) still
equals NULL. But a=NULL is true. So it didn't work out.

What is the best way to write a query and get the planner to use indexes
when you have optional parameters and columns that can contain NULL values?