I'm not the greatest bowler, but I offer some advice based on what works for me.
Hitting your target and most other problems are caused by one thing: Your mind is stuffed.
When you get up to bowl, what is more important to you, hitting your target or getting the ball to the pocket? I bet you might think hitting your target is, but in your subconcious you want to get the ball into the pocket. If you were using a cricket ball instead of a bowling ball I bet you could get pretty close to the target. You wouldn't be worried about getting to the pocket and would swing nice and straight.
What I do sometimes to help me hit my target is pretend the headpin is just right of the tenpin (in line with my arm and target) so I can swing towards my imaginary pocket without crossing my body. When I get really bad, I bowl through the middle arrow (from about 35 board) so that at the end of my approach, I am on the left of the head pin and I dont cross over my body. Just leave a lot of 5 pins if a weak backend. Then I move back out once my swing is sorted.
If that doesnt work for you or you want something more physical to change, I find these two things help me hit my target better while also giving me more length and stronger backend:
1: Shorten your approach and shorten your backswing. If your backswing is out of control at the top, how are you meant to get to the line consistently? Although you won't have a big swing, you can apply plenty of power without losing control because you should now have strong balance. (Also works with golf for me)
2: Loft the ball just in front of you. It is very hard to loft the ball while swinging across your body. Great for dry lanes when no matter how hard you try you cant get near your target. If all you want to do is hit your target, after lofting a few times you should be able to bowl normally again and hit your target still.
These are things I have discovered work for me but I can't say they will work for everyone. I hope they help someone anyway.