Took me 1 year to get my ave. over 200. Like everyone else says, it is spares that make your average. I look at it like every single pin spares costs you 11 pins per miss. If your making one extra single pin spare per game and you were averaging in the 190's. That spare will put you over the 200 mark.
Besides that, being able to change lines when you can't carry is also the next big thing. Too many bowlers will just keep on pumping it down the 10 board and live with what they get. Establishing a B game that you can go to will help you get better. If your versatile enough, go as far as establishing a C game.
I, like most house bowlers like to play the 10 board, but I can also play down the 5 board or go inside of 15 when I need to. I don't like playing 5 or inside 15, but at least I can. And for the most part I can score there too if conditions allow.
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As with all things, this is my opinion. And we all know, opinions are like butt-holes, we all have one.