transferring iphone apps from one mac to another

we have several macs, which means several apps on each mac, but don't want to erase the apps off our iphone/ipod touches each time we want to download something new on a diff mac. i tried copying the apps from one mac to another with a memory stick, but it didn't work... tried senuti too, to no avail... any recommendations on how to do this? it's frustrating having each mac say they will erase everything every time, and i don't think i should have to buy the same app for each computer (this goes for the free apps too).... help would be appreciated...renee