Truth box #1: The METRONOME
The metronome is the first gadget you should buy to assist you in your practicing. When choosing a metronome, here are a few features to look for:
See also: "Living with the Truth Box"
It is now possible to get an excellent quality tuner for a very reasonable price. While using a tuner, it is important to remember that you should use the tuner to help make your ears more sensitive. Sometimes musicians get hooked on the tuner, staring at it constantly to make sure they are right in tune. Often they get into the bad habit of constantly fidgeting with their pitch to the detriment of their tone quality. Remember, if your tone is not centered it is unlikely your pitch will ever be consistent. Don't sacrifice tone for pitch!
See also: "Intonation"
Truth box #3: THE RECORDER
Recorders are becoming more advanced and less expensive. If you can't afford a high-end recorder, even an inexpensive machine can show you a lot. With cheaper machines, do watch out for automatic volume compression.
Solid state recorders are the best option these days. There are a number of machines that record directly to flash memory, allowing for easy copying to a computer and easy editing. Two good brands are Zoom and Edirol.
I also enjoy using an older style mini-dictation machine that uses tape. I like this unit because I can play back recordings at half speed. This kind of playback reveals many small details, especially with the quality of attacks. Even though there are digital means to play things back at half speed, I have thus far not been happy with results of this technique.