Well, if you're going for the look-good-in-a-singlet approach, just do bench presses with high weights, low reps, you know the drill.
For actual strength, try laying on your stomach on an excersize bench and let your arms hang down with weights, and pull them up until you cant anymore. I forget what they're called, maybe rows? Do with a comfortable weight, and do 3-4 sets of 10 reps. It gets your shoulder blade muscles really well, and helps you keep your arms straight forward and back, leading to a more stable stride and less energy loss from your hips.
Another good thing to do is do bench presses with a lower weight, and just do a bunch of reps and a couple sets, to build muscular endurance with your arms for the longer races so your form doesn't break down. Also do this with your back muscles.