The advice from Ray Cwik is excellent. First, a really good and careful collimation is in order. The next step is to get a good Barlow. A 2X, preferably an APO type, will really improve the higher power performance of your scope. This is mainly due to the poorer performance of most eyepieces with the broad light cones from fast (f5 and faster) mirrors. In addition, you will get better eye relief when using the Barlow. After the Barlow, then a shorter focal length eyepiece such as the 6mm you suggested is in order. Do not consider one of the variable power Barlows as these usually suffer from excessive internal reflections. An APO Barlow is definitely a good choice over the standard Barlow and you will see a real improvement in the high power images that your scope can provide.

Jarvis Krumbein