The key to succesful buff hunting is to take it slow, unless there's an
absolute need to shoot them down ASAP(Which, in the MA, is quite frequent, btw..)
Like many have mentioned, the best possible approach to target requires sufficient speed, angle, and altitude. Also, staying out of the tail-gunner's cone of fire is the most important thing of all. This is because;
a) most people are used to tracking/firing from the tail-gunner's position
b) being inside the tail-gunner's cone of fire is the most dangerous place to be, when attacking a buff. Most of the buff's defensive armament are placed to fire at something approaching from behind
c) every bit of firepower helps, the higher the range of lethality, the better your chances of survival when making the initial attack and final departure.
Also as many have mentioned, people commonly make the mistake of approaching from behind - which, is basically a place you don't want to be at all.
I don't like it when people say "only attack from the high front" because 99% of the time the bombers are hauling bellybutton at 250MPH and you're behind them trying to catch up. It's impossible to catch, overtake, turn around and then attack from the front in almost every case I've ever seen.
Despite it all, an attack from the frontal quarters IS the most effective and safe method of attack. Like Mustaine mentioned, if you want to bring down a buff you take adequate preparations for the job, and positioning yourself for an attack is very important.
Ofcourse, you can be flying in something like a Me262 5k+ above the buff formation - in which case, you have a good chance of succeeding in a high speed attack which more often than not will knock out all three of them in a single pass.
However, in most cases one must do whatever he needs to do to take up a position which will ensure success - flying ahead of the buffs, while it is time consuming, is one of them.
An alternate method of attack, is a 90 degrees "slice" - which works well if you are not patient enough to fly ahead of them. Fly ahead of them a little bit, and then turn and approach them at 90 degrees deflection. Having a powerful firepower (such as the Fw190A-8 with 2xMK108s) helps. Your primary target is the tail surface, most usually the vertical rudder. A good aim at 90 degrees deflection, and the rudders will fall off - minimum ammo spent to bring down such a large bird.
At any rate, like with all other thinfs, it takes practice. Even with practice, dealing with a buff pilot experienced in defensive gunnery is always a dangerous bit.