The electronic tuner is useful for finding tendencies concerning certain notes and fingerings, and supplying a reference pitch for general tuning. When it gets to the point of really playing in tune, it gets harder. We have the vertical tuning within the chord, and the horizontal tuning of the melody. An example, using a B in the staff (b'). If playing the third of a G chord it should be lowered about 14/100ths of a semi-tone, however, if it leads to a C (as part of a C chord) the B should be a tad bit high for the G chord. Good intonation = compromise. Always.
Practicing resultant tones is a great way to develop ears. One player plays a G (g') in the staff, the other an E (e') at the bottom of the staff. When the interval is in tune, sum and difference tones pop out in the player's ears--you will hear a pedal C (C) and possibly a c''--the sum tones are easier to hear on piccolo trumpets and the difference tones on flugelhorn. I've encountered in-tune sum and difference tones with a seven-piece natural trumpet ensemble, although rare enough to be freaky.
In other words, Rowuk is right--ensemble playing is the best way to learn how to tune intervals, and I like mouthpiece buzzing for the melodic aspect. Sit at a piano, sound a c' (middle C) on the piano, match it and play a C Major scale with your mouthpiece, testing your c''.
It takes lots of hours to improving tuning, nobody is perfect, but ear training with the instrument provides a whole bunch of low-impact physical exercise (good for endurance) and stretches our ears.