Seriously though ...
Get a tight bore barrel, such as a LAPCO, and make sure you always have a good paint to barrel match, such as LAPCO Autosprit shooting Marbz always works well.
There might be some mods to buy such as different bolts and stuff that will also improve efficiency.
what size tank are you using now? Get a Bigger tank like an 88/4500
that always helps, and I've heard that bigger tanks balance the Shoebox a little better than small tanks ...
-Micah