Hash Partitioning

 I find a lack of documentation on hash partitioning that can answer my questions.

What fields are best suited for hash parititioning?

I have a table that has 2 highly selectable fields.  The table will be partitioned by range on a date field in yearly increments and sub-partitioned by hash on one of the two following fields.

Hash field candidate1 has low cardinality but the data is skewed.

Hash field candidate 2 has high cardinality but the data is not skewed.

Both fields are part of the table's primary key.

Also, once I know the best hash partitioning candidate, how do I choose the optimal number of hash parititions?