Explain Analyze help

I've just completed my application and my customer is starting to
populate the database. I've put a table in where queries get logged so
that I can start pinpointing my optimizations to the most frequently
used queries. I've read the manual on the explain analyze statement
and I have one (which will probably turn into many) question.

Obivously, I have a lot of sequential scans going on, at about what
cost does it start to make sense to make an index? Or, should I just
put in a bunch of indexes and let the optimizer decide whether or not
to use them?

OK, so it was 2 questions.

Anyone? Bueller?

Mark