Do a search for the "ScareCrow" motion activated animal deterrent. It's a sprinkler with a motion detector. It's about $50 on Amazon.

I also put up posts at the garden corners, and run a line of tarred twine along the perimeter of the garden. I like to use t-posts so I can stretch the line really taught. From that, hang several aluminum, disposable pie pans, spaced out every 12 ft or so. Hang them in pairs so they clank together. When a deer walks into the line, the CLANK will scare them away.

Deer were eating some of my newly planted trees so I placed wire fencing on the ground around each tree. The deer don't like walking on it and have left the trees alone so far. As an added bonus, the critters that were digging for bugs in the mulch also don't like it.