Know the intervals and chords available to you very, very well. Like ssab7, the open D is your 7. A way to actually use this would be to take the other notes in an Emin7 -- let's say the root (E), 3(G), and the ocatve -- and find ways to play this while an open D drones. Keep it rhythmically interesting, too.
Also, explore your different modes, like MacheteJames suggested. If you're playing an Emin, other modes that might work would be your relative major (G) and your mixolydian (D) and the chords available from them (Gsus2 sounds really neat, especially playing the octave of the 2 and a D7).
Finally, don't be afraid of the upper register, but don't spend your whole time up there. A way that I like to leave the upper frequences without sounding choppy is to run (don't walk) down an arpeggio back to the "bass frequencies", do a couple more bars, and then end the solo there, comfortable and in position for the rest of the song.