optimizing Postgres queries

Hi,

we are new to Postgres and we are evaluating Postgres 7.4 on MacOS X as
an alternative to FrontBase 3.6.27.

From the available features Postgres is the choice #1.

We have some tests to check the performance and FrontBase is about 10
times faster than Postgres. We already played around with explain
analyse select. It seems that for large tables Postgres does not use an
index. We often see the scan message in the query plan. Were can we
find more hints about tuning the performance? The database is about 350
MB large, without BLOB's. We tried to define every important index for
the selects but it seems that something still goes wrong: FrontBase
needs about 23 seconds for about 4300 selects and Postgres needs 4
minutes, 34 seconds.

Any clues?

regards David