Shorter stem/Higher rise (in Theory) will put your weight more to the back which will make the front easier to pull up (im assuming for manuals etc?).
You need to watch you dont go too short though or it will be a dog on the back wheel and can be difficult to get enough movment/power to do larger moves.
I used to ride my Sidekick with a 110mm 30deg stem and it rode much like a trials bike, when i got a dedicated trials frame i put a 50mm 5deg stem on to the sidekick and it rode like a BMX, awsome for street stuff, manuals, large bunny hops, rolling gaps etc but its pants on the back wheel when stationary for gaps, back hops etc.