As leftside said, practice throwing straight at most spares, and devote a practice session to working on your spare shooting.  For example, I have spent most of a practice session doing nothing but throwing at 10 pins, both balls.  You can do the same with other pins as well.  If you can, don't even turn the scorers on, that way you are not worried about the score, and you can focus on what you are working on.  Treat your spares during practive with the same amount of concentration that you do on the first ball. Don't let up on them.

1st - buy plastic.  50 bucks for a spare ball can earn you a mint.

2nd - learn to throw said plastic ball at spares.  This is how I do it.

Game 1 - throw at the 7 pin 1st ball, to the pocket on the 2nd ball.
Game 2 - throw at the 4 pin 1st ball, to the pocket on the 2nd ball.
Game 3 - throw at the 2 pin 1st ball, ditto above.
Game 4 - throw at the 1 pin 1st ball, ditto above.
Game 5 - throw at the 3 pin 1st ball, ditto above.
Game 6 - throw at the 6 pin 1st ball, ditto above.
Game 7 - throw at the 10 pin 1st ball, ditto above.

After doing this, you will have fired at every area of the lane.  I don't recommend taking an entire practice session and throwing nothing but spare shots for a couple of reasons: you don't do it in league or tournament play, and it causes muscle confusion.  The first and only time I spent an hour throwing at only spares had me trying to recall how to get lift and turn on the strike ball.

Basically, you are bowling 'backwards' here by going for spares before you try for the pocket.
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Nut

If horse racing is the sport of kings, then surely bowling is a very good sport as well.