Next time clean off the grout before it dries with a damp sponge that you keep ringing out in a bucket of warm water. Let the sponge lie flat against the tile and lightly glide it over both the tile and the spaces with the grout.  The tile is raised up from the spaces so just let the sponge pass "lightly" over the spaces taking the excess grout from the face of the tile onto the sponge without taking the grout out from between the tiles. Keep rinsing the grout out of the sponge in the bucket of water until the grout is out of the sponge. Squeeze out most of the water and keep going flat over the tile with a rinsed out damp sponge until the tile is clean and there is only grout between the tiles. If you want this messed up job to be really nice you probably need to do it over. This is what you do depending on how big the tiled area is.  If it's a small area on only one wall get a piece of sheetrock or cement board and cover over the messy tiles with it so you have a fresh wall. You might need to drill through the messy tile first and then nail or screw the board to the studs by putting the screw through the hole you drilled. Don't drill too deep, only enough to get through the tile; the screw will go through the other wall and into the stud. Then tile over the fresh wall with new tile and grout it the way I explained above. One other thing:  don't grout the whole wall at once; grout and clean smaller areas before moving to the next area. It's more important to get the grout off the face of the tile before it dries than it is to grout it all in one shot. You can always go back and add a little grout here and there if you miss a spot; it's a lot easier to add wet grout to fill spaces than it is to take dried grout off of tile without damaging the tile. Unfortunately there is not a sure way to remove dried up grout from the face of tile and get a professional looking job; you have to do it over... Sorry maybe someone else has a better way but this is the way I would do it.