Fly a 1-2K over the bomber, and slightly infront. Roll inverted, dive down.
Head-on passes at a slight angle tend to be highly effective as well, if you can get the aiming down. It seems the deflection and high closure rate rattles many gunners.
For guns, I like the 30mm's best. 1-2 well-placed rounds, and you've got a kill. However, .50's will work fine if you've got more than just 4. 4 will do in a pinch, but you'll have to hold down the trigger noticably longer.
20mm's are also great. Still have to concentrate fire on a point, or a specific part at least, but still very easy to use.
Personally, I don't find that the 37mm's offer a big enough increase to justify the reduced ammunition load vs the 30mm's. That and anything mounting 37mm's has inferior preformance compared to a fighter with 30mm's.