How often should I have my tank cleaned out?

Depending on the amount of people living on the system. 

     1-2 people 3 years
     2-4 people 2 years
     5 or more Yearly

The tank needs to be cleaned to remove the solids before they build to a point where they will start flowing into the drainfield and cause damage which will lead to a costly repair.

Do I need to add bacteria or other additives to my system?

No, The bacteria that is needed in your system comes from the waste stream.  In some cases the additives can actually harm your system by breaking down solids and allowing them to flow to the drainfield which can cause costly repairs.

What kind of effect can my washing machine have on my system?

A HUGE ONE!  Doing over excessive wash can overload your system and cause a failure.
Always spread your wash out across the week to allow for maximum absorbtion.  Always use liquid detergents rather than powders.