This is a drill that I learned from Bert Truax that really got my multiple tounging on the right track. It can produce results very quickly. It originated with Joseph Alessi, Sr. and can be applied to any exercise, etude, excerpt, or solo.
1. With the metronome, play the line at a comfortably fast single tounge.
2. Play the line again, one note to the beat. (For example, 16th note equals 92). Alternate, soft on the "ta" and loud on the "ka." It is important that the airstream remain steady and the notes connected.
taKAtaKA not ta KA ta KA.
3. Playing the notes at an equal volume begin to increase the tempo until you have surpassed your original single tounge tempo. One big advantage to this approach is that the gap between single and multiple tounging is automatically elminated. One caution, this drill can be very tiring the first few days, use common sense and know when to stop.