Optimizing a query through its syntax and indices

Hello,

I talk PostgreSQL 6.5.2 ...

Suppose we have some tables we join using some of their fields.
Suppose there are "restrictions" for the values of some (or all) of the tables'
fields of this kind:
tableM.fieldN [ NOT ] IN (constA,constB,constC),
tableP.fieldQ [ NOT ] IN (constD,constE,constF),
etc...
These restrictions can occur for the fields by which we join the tables, as well
as for fields of tables we do not use for joins.

Are there (can there be) general ( or not so general :-) ) rules for optimizing
the query ( and the performance and the speed ) through the particular order we
put the join statements in the WHERE clause, trough mixing/ordering the join
parts and the restrictions ( in the means defined above ) in the values of the
fields in the WHERE clause, through changing the syntax ( for example, using
UNION or EXCEPT clauses ), or through using one- or more-than-one-field
indices, or through doubling some of the restrictions if they refer to a
field we join by ( for example, "... WHERE tableM.fieldN = tableP.fieldQ and
tableM.fieldN in (constA,constB)" complemented by " and tableP.fieldQ in (constA
,constB)" ).

I hope I was clear enough for my question to be understood.

Any help will be appretiated.

Regards,
Stoyan Genov