As others have said, using a spare ball is paramount if you want to have the greatest success over a variety of conditions. Plastic can start to crack while urethane will be longer lasting. Plastic however will react less to friction than the typical urethane so there are pros and cons to each. At least plastic is cheap enough to replace whenever needed.
And as other have said, practice makes perfect. There has to be some muscle memory involved to the point where shooting at a spare instills confidence.
And lastly, and I believe this is an area where not enough attention is spent, is HOW to pick up particular leaves. If shooting cross-lane for corners, how much cross-lane to provide the most room for error? How to approach the bucket without/with the back pin. How to approach washouts, various two pin combinations, 3-6-10 with and without the 9. Too many times I see bowlers approaching common spares in uncommon manners not giving them enough room for error.
Once you have a system down for each spare combination, make a list of where you stand and where you roll the ball. This is why having a spare ball is paramount since your system will/should be about the same on every condition you approach. This will instill confidence in your spare game which in turn will allow you to be more successful. For instance, no matter the condition, I will stand with my right foot on 37 and roll my ball over 20 for the 10-pin. Works wherever I go and allows me to not worry about the condition on that shot, along with other leave combinations.
