I can't seem to get a straight answer when I call the USPTO
The major milestones for you to track occur in the fifth and ninth year after your initial registration. Between the fifth and sixth year, you are required to file a “Section 8 Declaration of Continued Use” which is a sworn statement by the owner asserting that you are still using your trademark in commerce.
Alternatively, if you are not using your trademark at that point, but want to prevent abandoning your rights to the trademark, you can file a “Declaration of Excusable Nonuse,” where you swear to the USPTO that you’re not using your trademark due to an excusable reason. Not using your trademark just because of decreased sales is NOT considered an acceptable / excusable reason. Nor are delays in using your trademark in commerce due to ongoing negotiations with distributors considered excusable. Acceptable reasons for nonuse include: a trade embargo prevents you from using your trademark, interruption due to retooling of a plant or equipment, or an illness or catastrophe. Keep in mind you’ll have to give the USPTO a date when you expect to start re-using your trademark. Your Section 8 filing costs $100 per class of good/services. See: http://tess2.uspto.gov/tmdb/tmep/1600.htm#_T160411.
Because the term of protection under the UPSTO for a trademark is 10 years, between year 9 and 10, you have to file a “Section 9 Application for Renewal” along with the Section 8 filing described above. The combined fee for both Section 8 and 9 is $500 per class of goods/services. If you fail to file these, you risk having the USPTO deem your trademarks abandoned and will have to re-start the entire registration process over.
Finally, you should consider subscribing to a trademark monitoring service to ensure that others during their application for registration, aren’t registering something too similar to your trademark.
Note – this is not legal advice, and no attorney-client privilege is established.