Once you have a federally registered trademark, and are a proud recpient of the certificate complete with gold seal, your job is not done. There is periodic upkeep, such as filings with the Trademark office between the 5th and 6th years. It is important to continue to use the trademark in commerce, as abandonment can be found from lack of use. If you expand the good or services you provide under the trademark, consider filing a new application for them.
In addition to the above, you should, at the very least, make regular searches of the USPTO Trademark database to determine if any competing trademark applications have been filed. While the USPTO will regularly refuse to register what it believes are conflicting trademarks, it doesn't catch everything. Early contact with the owner of the proposed trademark may avoid a costly dispute.