I used to bowl medium paced out swing and the ball that always got wickets was the off break, ripping my pointing finger down the seam, two or three out, and one cutting back in really worked! the success was far higher than trying to ball a faster delivery, so I worked and worked really hard on my off spin, and have a quicker medium pace ball as a rare variation.
Watch the batter closely when you aren't fielding, look at his grip, hands too far apart could help a slower ball be hit high for an easy catch.
Keep it tight, don't give your skipper an easy excuse to replace you.
I love the cat and mouse game with a batter, but remember that batters are very rarely out to a good ball, it is when they lose concentration or go for a big shot, good balls are played with respect and are usually defended.