Sometimes when your windshield wipers aren't doing a great job it might be time for new ones. However, other times they might simply be dirty and need a good cleaning. When that happens, you can try using a glass cleaner to give your wipers a good scrubbing. Just add some glass cleaner to a soft rag and proceed to clean your windshield wipers. If you find that a lot of excess dirt is coming off then that might of been why your wipers haven't been working great.
The glass cleaner will also help as it wipes your windshield down. Optionally, you could add something like Rain-X to help your wipers remove even more water and make sure you're safer the next time you have to drive through heavy rain.