Japanese beetle traps are usually ineffective for controlling the beetles. The pheromone scent in the trap actually attracts more beetles into the area. If you can persuade neighbors a block or two away from your yard to install the traps, you might see fewer in your yard. All the beetles will fly to your neighbors! For long-term control, milky spore will be helpful, but you may not see results until the following season, because the spores work at killing the grubs that turn into beetles rather than the beetles themselves.
Japanese beetle populations can vary dramatically from year to year. It's usually best to use milky spore in combination with other treatments. If your garden has only a few Japanese beetles, kill them with soapy water. Go out early in the morning with a small bucket of soapy water. Hold the bucket under a flower or branch where Japanese beetle adults are resting. Tap the branch; the beetles will drop off into the soapy water, where they'll be killed. If you have a lot of beetles, you can spray with an insecticide such as carbaryl (Sevin) or Malathion.