How to clean out the gutters while still on the ground

Yes, the pressure washer will dig out the sand between the bricks. I would not recommend it.

I would suggest a stiff brush with dish detergent mixed with water. If that is not strong enough try some professional paver cleaning products from your local home improvement store.

I have cleaned gutters at multiple one-story houses by kneeling down from the roof, but I wouldn't risk it on a two-story house.

If you have a two-story house or are wary of cleaning the gutters from a one-story roof, then you have two options:

1. As BMitch has said, get a stabilizer bar (~$25-75; I recommend this one as it is easy to adjust and has no nuts and bolts that you can accidentally misplace) and an extension ladder (if you are a bit on the heavy side, get fiberglass; fiberglass ladders are heavier but stronger.) This will make your ladder more stable, as ladders can very easily slide along the gutters, leading to a fall. Despite this additional stability, you should avoid becoming too confident and leaning over instead of getting down and moving the ladder. Remember to slope the ladder correctly. The easiest way to get this right is to put your toes against each foot of the ladder and put your hands out, straight forward. You should be able to touch the ladder, but just barely. When moving a ladder that's already set upon the roof, it's easiest to stand underneath it, between the base of the ladder and the house. This keeps the ladder's center of gravity above you. Watch how this fireman lifts the ladder, starting at 2:56. Pause the video at 3:01 and imagine that the building is behind him, and all he would do to move the ladder at that point is to take a few steps to the side and place it back down.

2. Another option is to remove the material from the roof, wearing a harness for fall protection. You will need a harness, lanyard, rope, and something to attach the rope to. You can get all of this for $100-200. (Example) Make sure you wear the leg straps pretty tight, as they can slide up into and damage your thingamajoo if you fall when they're loose.

As far as what tools to use, whatever gets the job done. Hands (gloves recommended), small hand shovel, or maybe something like this. A broom will also work. If you're working from a ladder, you should reach for a tool with a short handle. If you're working from the roof, you should get something with a long handle. If there's any sand/dirt buildup in the gutters, hit it with the garden hose.