When in bombers, open up your Clipboard. There is a Menu called E6B, this is a 'flight computer', you 'll want this open. It'll let you know speed, climb rate, and time left at your current fuel burn rate. When lining up on a target, make sure you're on a mostly straight heading for your target by 1 sector out, and make sure you're level.
Once your Climb rate registers as 0, you are at even altitude and even speed. At this point hit F6 to go into your bombsite. Hit "U" to enter calibration mode and hold the "Y" key for at least 2 seconds (the longer you hold it, the more accurate for the most part). Once calibrated, hit "U" again to go back into bomb target mode. From there, try not to adjust course too much. Any corrections will change your climb rate/speed and affect calibration.
Put your + on the target, and try dropping just before your + hits the target. You'll be blowing crap up in no time.