Refining query statement

   Working with my sales/client management system using psql I have a select
statement to identify contacts to be made. This statement works:

select (C.contact_id, C.lname, C.fname, C.direct_phone, O.org_name, 
A.next_contact)
from Contacts as C, Organizations as O, Activities as A
where C.org_id = O.org_id and C.contact_id = A.contact_id and
       A.next_contact <= 'today' and A.next_contact > '2018-12-31' and
       A.next_contact is not null;

but would benefit from tweaking. When I have had multiple contacts with
someone I want only the most recent one displayed, not all, and they should
be no more ancient than a defined period (e.g., a month).

   I want to learn how to make this query cleaner and more flexible. When I
write the UI for this I want to be able to specify a data range in addition
to a fixed 'today'. Pointers on what to read will be very helpful.

TIA,

Rich