Apple says users who "consistently encounter issues connecting to a specific Wi-Fi network" should update their router's firmware. If you happen to be using an AirPort Base Station, check out this article for details on performing the update, otherwise the information should be readily available at your device manufacturer's website.
After updating your router, you should make sure you're using WPA or WPA2 wireless security, and not WEP. Next, you should renew your IP address by going to Settings > Wi-Fi > select your network > tap Renew Lease. Other suggestions include toggling Wi-Fi on and off, as well as ensuring that your screen brightness isn't at its lowest level (Settings > Brightness & Wallpaper).