As stated above, do not use a relay for every machine only. Instead, use a filter or a sorting machine to enter machines into one huge loop. Inside that loop, redstone tubes touch the machines, inserting the items and the rest that has no place yet goes to a relay inside that loop WITH A RESTRICTION TUBE right where the items enter the relay. This way, all the items which went through the relay try to enter a machine again.
And for whoever says that I don't have to make a loop here, REDPOWER SYSTEMS CAN GET JAMMED. I don't want my entire tube network to get jammed, but one small part of it ( still it is as much as possible prevented that it jams up with this ).