Repeat the 8 row rib and cable pattern (probably mostly rib with the cable crossover every so many rows). The decreases will take away stitches from the rib pattern (and may eventually eat into the cable pattern). That's okay, just continue the rib with one fewer st on each end. So instead of k2p2k2p2k2 for example, you may have k1p2k2p2k1 for a few rows and then p2k2p2 after the next decrease as you lose edge sts. You want the columns of rib to continue up the sweater.
Can you give us a pattern link or the name of the sweater?