Washington, while living in Virginia, managed to manumit several of his slaves during his lifetime, and the remainder of them on his death - if Jefferson had felt as driven as Washington, he could have done the same.
Jefferson’s anti-slavery efforts include:
1. Introduction of a bill in 1769 the Virginia legislature to abolish the importation of slaves into that state.
2. Inclusion of an anti-slavery provision in his original draft of the Declaration of Independence in 1776.
3. Initiated the Congressional ban on slavery in all federal lands in 1784 (his effort to extend the act to the 13 states lost by only one vote).
4. In 1808, as President, he signed into law a bill banning the slave trade with Africa.
While Jefferson did not free all of his slaves on his death (as did Washington), a law passed in Virginia in 1806 required that the legislature pass a special bill that would attest to the exemplary behavior of each slave to be freed. If freed, the slave had to leave the state without his or her family. Jefferson was not in favor of this law. Further, Jefferson trained his slaves in skills that would be useful when they were free. He believed that to free them first would be irresponsible — since they would be homeless and without family.
http://www.liberty1.org/defense.htm