gtromble

1 minute single-tonguing exercise -- a la Chris Gekker

mid-staff G
metronome on, say q=80
play 16th notes for 1 minute, breathing when necessary (skipping a couple of beats is fine, just take a relaxed breath and start again)

make the notes consistent in articulation and sound

at some point during the minute, your tongue may tire so you can't keep up
if you're good at that tempo, skootch it up the next day

once a day, every day
you'll be amazed at how it improves your playing - just don't push the tempo -- it isn't a race -- it's more about the consistency of a good articulation