The RSPB puts the declines in song birds down to intensive farming practice. Magpies are one of the very few species that have actually managed to adapt (because they are rather smart) so one should not blame them.
It is not true either that one can shoot magpies at any time. Magpies are protected by law (Wildlfe & Countryside Act 1981) and anyone killing or trapping them illegally faces fines. You can only trap them under licence.
I was told about an 'experiment' where magpies were trapped at a nature reserve in the hope that song bird numbers would increase. It did not work at all, without the magpies 'less attractive' bird species simply dominated.
It is often all too complicated for us to come along and mess with it. There has been some success at protecting ground nesting birds at reserves, but we don't get too many of those in our gardens so it is unlikely to help with birds in a garden. Other research has shown that it is the magpie population that relies on the song birds, removing the predator makes no difference at all to the prey species popoulations. What song birds are really missing is the ample food that was once in the countryside, weed seeds, insects, spilt corn, berries in the hedgerows.
Rant out of the way, bird boxes can take up to three years or more Gardening Grandma to get any interest. You could try putting in some straw at this time of year and see if birds use them as winter quarters. They might then think of nesting in them.
Sometimes when feeding the birds just trying a whole range of different things is the key. Often neighbours are feeding them already and many of the birds will be spoilt for choice. I have a friend who's birds never touch the peanuts, at my table the little guys are sat in the bushes waiting for me to put them out each day, but if I put out mixed seed it is never touched at all!
I don't put out bread as it tends to bring in mobs of starlings, but even they are declining so I might reconsider that. I find too that the magpies and jays at my table tend to come in at different times. They have their fill early on and the smaller birds come later.