I've been thinking lately that speed and stamina are what I should be working on. I feel like my lack of speed and stamina make certain 8th note passages harder than they ought to be. To that end, I've written myself an exercise.
Here's the link.
Step 1: Find a tempo where I can play eighth notes on a single string comfortably for a long time
Step 2: Set a metronome there, and go through the exercise once as a warm up.
Step 3: Raise the metronome speed a few clicks and repeat.
Step 4: When I get to the tempo where I can't get past exercise E, park the metronome there and repeat a little bit.
I recommend doing this at the end of practice, rather than at the beginning, because it will wear you out.