How to handle things that change over time?

I am working on a system that will support internal bossiness work for a
company. Periodicly things will change in their "world". For example they
periodically recalculate the "mark up" of various components, of their
bushiness, such as labor and or purchased materials. Presently I am keeping
these constants in a table, and I have the constant, and an effective start,
and end date for these factors. In this scenario, the number of rows will
just grow over time, and by using the proper conditions in our select
statement, i can apply the correct factors as they change over time.

In another scenario, a column of the employee table is the pay rate.
Obviously this will change over time, also. It does not appear to me that
this lends itself to this same treatment, as most of the data associated
with a given employee, is fairly static, and if I add an entire row, just
because the pay rate changed, this looks overly complex.

This cannot be the first time this issue has been addressed. What have
others done in this scenario?