I've been flying the 109K almost exculsively this tour and I have have pretty good success rates against enemy bombers.
The 109K performs
magnificently at higher altitudes, so it's quite good for high-level bomber intercepts, especially if there a P51s stupid enough to get in your way at that altitude.
I can't add much to what has already been said here, diving/slashing fast attacks are definately the way to go and with the K4, you have such an outstanding climb rate that it's really no bother to put a couple of K on a set of bombers and then drop in hot and heavy on them.
With B-17s...be carefull, the guns (especially tail) are very vicious.
With 24s, as was stated earlier....go for the wing root... this will result in either an engine fire (fatal hit) or a wing seperation (also fatal and much cooler to watch)
Lancs.... belly shots are the best bet. I always try to take the tail section off too with those.
I thought I'd post a couple of photos of the 109K in action against bombers.
I think this is a good example of a front quarter, diving, slashing, front-on assault.
This sort of attack will (most importantly) make you very hard to hit and as you see in the photo, inflict damage on engines and the cockpit.
Not one round got near me on this pass and this pilot was an exceptional gunner.
This second image is a perfect example of the B - 24 attack that has been mentioned earlier.
Here, I took the time to climb +2k on this guy and dive at high speed (i came in from and angle and straightened up at the last second so as to make me very hard to line up for a shot)

I destroyed all three bombers in this flight (the pilot was no dead**** either) with a few 30mm rounds to spare and with nothing but a scratch on my canopy.
Hopefully, these shots will help you size up and perfect your attacks a little to.
Oh, i just rememered. Do youself a large favour and look up the movie by 'ParIN' (pretty sure it was him) that he called 'Skull'. He posted it here not long ago, there are some great 109 techniques in there.
anyway, enough jabbering!
Good luck!