best way to choose index to cluster on?

I am looking to squeeze a bit more speed out of my database by clustering
most of my tables. However, on tables with multiple indexes, how is the
best way to choose the index. I am thinking I want to use the index with
the largest pg_stat_user_indexes.idx_tup_read. Would this be correct? If
not, what column(s) and views should I be looking at to find the most
popular index?

Secondly, I have some partial indexes on a very active table, and it tends
to have the highest idx_tup_read for that table. Is is possible to cluster
on a partial index?

Thanks,

Chris