By sticking a cloth or a soft sponge under the strings near the bridge, you can turn your guitar plucks into little thumping sounds. Limiting the resonance of the guitar to these not unpleasant thumps refocuses your attention on two very important qualities necessary for great tremolo technique: rhythmic evenness from thump to thump, and the quality of intensity of each thump.
So put a sock in it:
Hope that helps! Stay tuned for Part 4.
For more active practice techniques, check out: Mastering Tremolo