For a better download experience:

Turn off all other internet applications.

Have your computer dedicated to your download only. Depending on your computer's hardware and your connection, multi-tasking while downloading may extend the time it takes to successfully download your title. 

Stop other activity on your internet connection.

If you have more than one device active on your internet connection, it is recommended to not use them while the game is downloading. This includes downloading of music or movies and playing other games (e.g., World of Warcraft).

Make sure windows is up to date.

Using Windows Update will make sure your Windows operating system has the latest in security updates and drivers. 

Update your hardware drivers

as recommended on your hardware manufacturer's website. Even modems and network cards have updated drivers that may actually improve the quality of your internet experience by performing more efficiently.

Process one download at a time.

Your connection does not give the same speed to all downloads. If you have a 1 meg connection, it will split it between downloads (it will not give 1 meg to each file).

Check your router documentation

for any settings that need to be configured for downloading.Some routers may require D2D to be added as a trusted site to download successfully.