I usually work on timing, and hand release, as those are two areas that I can struggle with during the course of a season. The best thing to get out of a practice session is a feeling that you have corrected some sort of fundamental flaw that is causing you to be practicing in the first place. I do not make it a habit to practice as much during the league season, as I bowl on three leagues and use those as practice for tournament play, but if I feel there is a glaring weakness in my game that needs immediate attention, then I will practice to get my game back in order. You should try and videotape yourself and then study the footage, as it is a valuable tool to have when you are trying to correct mistakes in your game. Most of all, have fun with your bowling and do not always over analyze every session of league or tournament play. There are times when you simply have a "bad day", and by over analyzing your game by going out and throwing 20 practice games, you can get into some bad habits that are even harder to get out of. If you notice the same mistakes two nights in a row, then correct it, but if you bowled a 540 then backed it up with a 750, chalk up the 540 to a bad night, and move on. I hope this helps, just my two cents, good luck and good bowling!!
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MaineLefty