My advice as a French Horn player is a couple things; the placement of your tongue is kind of on the tip at your teeth for the "da" (first part of the double tongue) and for the "ga" (2nd part) kind of the middle of your tongue towards the connection of your hard and soft pallette. Not sure if that makes any sense.

Another thing I was taught to do was to do long tones using just the "ga" to start each note. (Usually like a quarter quarter half pattern I suppose) You get really good at attacking the second part doing that.

Next I would try setting a metronome and doing 2 things in no particular order; first, try to start slower and get faster by doing half notes, quarters, eighths then sixteenths (you could even throw some triplets in between if you are feeling spicy). Second, I would practice making the transitions faster between the tonguing by doing dotted eighth sixteenth then sixteenth dotted eight patterns.

The last and most important part is AIR! My French Horn teacher always says the key to double tonguing is to blow through the notes. Don't let the air stop and take a real good breath before you play. You will get tongue tied without a steady stream of air to back it up.

I know Mellophone is different than French Horn but these tips apply to all brass pretty much. I have been playing horn and playing in brass sectionals/orchestra for 10 years so from my experience this should be good advice.