An exercise I use with new folks is to have them sit in ki-za (seiza but with "live toes"); I have them position one knee slightly forward to make hamni, then lean over to touch the leading shoulder on the ground, extending the arm across the line of the body; look up at the ceiling and point your other arm up in the air, extend your rear leg and stretch it up and out until you roll from your shoulder to the opposite hip. This exercise let's you feel a nice, safe line to roll (shoulder to opposite hip).
If you're having trouble from the standing position, try making sure that your front knee (in hamni) is bending as you push/launch into ukemi and you are projecting forward, not just trying to flip yourself over in mid-air
If someone tries to hurl you across the mat into front ukemi, try sinking into your knees during the technique, should make the technique slow down and become heavy, which will give you a little time to prepare for the ukemi and also start to wear your partner down if he's muscling you, make him tired and less able to use so much muscle.
Lastly, grab a senior student who takes really nice ukemi and bribe him/her with beer for extra pointers on how to take safe ukemi. Observe and mimic the smoothest uki on the mat - remember, quiet ukemi is generally soft ukemi.