A few general things about practice...
Don't keep score. The numbers are the last thing you should be concerned with. If the scorers have to be on, punch in all the games at once and hang a towel over the console.
Always warm up first. Throw a few shots to get loose and guage how you are feeling physically.
Get a coach that you trust to tell you what to work on, and how. There is nothing worse than training in poor technique.
Have a clear idea of what you are working on and why, and have a clear goal for each training session. It may be as simple as hitting a target with 10 consecutive shots, holding the followthrough consistently, or whatever.
Be honest with yourself, both with your progress and your expectations. Don't expect to add 30 pins to your average overnight, but accept that if you are not improving you might need some help from a good coach.
Think about every ball you throw, and don't waste any shots. Throwing strings of strikes looks good, but halves the number of shots you get per game. Practice is expensive - get as many shots as you can for your money.
After each shot, don't move - watch the ball across your target, check your follow through, then your foot position (where you slide on the approach) shoulder alignment and that you are solid at the line. This will pick up a lot of footwork and timing problems such as late or early timing or excessive or inconsistent drift in the approach. (If you can't hold your position at the line long enough to check these things, then you have a major timing problem, so fix that first.

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Lastly, just like league or tournaments, sometime practice goes well, sometimes not. Don't beat yourself up over a bad session - it just gives you something to work on next time.
Cheers, Robbie.