What about a solar sail mixed[1] with, ion engine[2] and nuclear powered[3]? With all those things combined, you would constantly be accelerating for many years.

[1] http://www.planetary.org/explore/projects/lightsail-solar-sa... [2] http://www.nasa.gov/centers/glenn/about/fs21grc.html [3] https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_ge...