One important thing with bombing I've found, and Dave mentioned it, was make sure you're all levelled out and your speed is constant and you're on the right heading at AT LEAST 2 GRID SQUARES AWAY from the target. I'm a crap multi-tasker, so I prefer to be setup at 3 grid squares away.
Once I get to the right altitude, I throttle back to 80%, so that I don't get my airspeed creeping forward after I've levelled out from climbing.
That means you have all your settings dialled in correctly to match your height and current TAS. All you need to do now for 3 grid squares is minor rudder trim alterations in order to stay on target.
If you leave this too late, you'll be trying to level out, whilst also entering height & speed and trimming and etc.. and it's a nightmare.