Patching holes can be a pain and rarely do we get the desired result, so here are some tips for a professional finish.
Always remove all loose or flaky paint from the area and sand any bumps until they are flat before going anywhere near it with filler.
For small holes filler should be used in at least two applications. The first application is to seal the hole and should sunk slightly from the surface line, this should then be left to dry completely before a quick sand with some fine grit sandpaper and a wipe with a damp cloth to remove most of the dust.
The proceeding applications should be applied using a spatula and pushed into the cavity. The final application (if more than two) should be smoothed over leaving the filler very slightly proud of the cavity. A smooth finish filler is best for the final application.
Once the filler is completely dry it can be sanded back using a fine grit sandpaper. It is important to use a sanding block at this stage and not to over sand. The block will help to keep the sanded filler flat with the surrounding area.
Large holes in plasterboard need to be reinforced before filling. It is usually good practice to cut out any cracks in plasterboard even if it makes the hole bigger. A neat rectangle is best to work with as you will be able to cut a new piece of plasterboard easily to fit into the hole, note that when cutting plasterboard with a Stanley knife the paper will raise slightly along the cut, this will need to be flattened in order for it to fill smoothly, a good tip is to roll the edge with the side of a screwdriver.
A larger 'plate' piece of wood will need to be placed behind the hole and screwed into place through the existing plasterboard using plasterboard screws, make sure that the head of the screw sinks in slightly to the plasterboard for filling. The new piece of plasterboard that has been cut to fit into the hole can then be screwed in place to the same wooden 'plate' piece.
Using the same method as above for the small holes you can bring the wall back to its original state but remember to use the sanding block each time to keep it flat.
If you are repainting the entire wall then you will get a good even finish using a roller.
If you are simply touching up the paintwork then try stippling the brush on your final coat to match the surrounding paint, which most likely was done originally with a roller.
However you choose to paint, before you hang your pictures consider the use of a picture hanging system which will eliminate the need for repeated drilling and filling in the future.