If things keep falling off your wall shelves, there are two basic ways to go about fixing the problem. The first is to change or correct the way that the shelf is installed. If the shelf is properly installed, then you can add some small pieces of trim to the shelf to keep objects from sliding off.
Things You'll Need
Screwdriver or electric screw gun
1-1/4 inch wide lath strip or strip molding
16 oz. claw hammer
1-1/4 inch brads
Securing the Shelves
Use the 4-foot level to see if the shelves are level. If they are not level, you will have to remove one or more shelf brackets and re-attach the brackets to the wall so that each bracket lines up in a straight line and the overall alignment of brackets is level. If the shelve is badly out of alignment, remove all brackets except one and use that one bracket as a starting point for re-establishing a level line of shelf brackets.
Check to see if the brackets are level in a direction that is perpendicular to the wall. In other words you want to take the two foot level and run it directly away from the wall and across the width of the shelf. This plane needs to be level as well as the one that runs along the length of the shelf.
Make sure each shelf bracket is screwed tight to a wall stud and not just inserted into the drywall or plaster. If the bracket is not tight, it will have to be re-mounted or moved to one side or the other in order to find a wall stud. If the situation warrants that you must remount the shelf bracket to the same spot in the wall, you will need wall anchors to do the job.
Add a Piece of Trim
Take all items off the shelf and take the shelf down and place it on a set of saw horses. If you have the type of shelf brackets that curve up and hook on the front end of the shelf you will have to take 1/2 inch off the width of the board. If not then you can leave the board as is. Make the cut with a circular saw and a chalk line to mark the cut. After the sliver is cut off the board, you can smooth over the cut edge with a block plane.
Use the backsaw to cut a piece from the strip molding that is an inch longer than your shelf boarding. This measurement depends on your wooden strip being 1/2 inch in thickness. If the strip molding is 3/4 of an inch in thickness then add 1-1/2 inches to the cut.
Nail the strip of wood to the front of the shelf board. Be certain that the bottom of the strip molding is flush with the bottom edge of the shelf. Also make sure you split the overhang at each end of the shelf board. This means you will have an extra length or either 1/2 or 3/4 inch at each end.
Measure the two side pieces of strip molding that will go on each end of the shelf. They should be cut at the same length as the width of the shelf.
Cut the two side pieces and nail them to the sides of the shelf. Be sure the bottom of the strip molding is flush with the bottom of the board.
Put the shelf back in place.
You may want to paint or stain the strip molding in Section 2 to match the shelf.