by the main phone connection to the house (a large BT socket)
next to a power point (ideally you should use a surge protector,
and you can often get these with extension leads)
somewhere near the middle of the house/flat to spread reception
equally (or perhaps slightly closer to the garden if yours is quite
large)
on the floor of the first floor of a 2 floor property, or on the
middle floor of one with more than 2 floors, again to spread reception
equally
Of course, it's not always possible to get all four. The first two are
most important.
Filter
Those filters are important. Broadband (ADSL) signals are carried on
the same wires as phone calls, and need to be filtered out at each end
- at the exchange and at your end. Filters need to be installed so they
form a "barrier" between the Livebox and everything else that uses the
phone line.
The simplest way to do this is to plug the filter into the main phone
box, the Livebox in the ADSL plug and everything else (phones, faxes,
Sky boxes etc.) into the phone socket. However, many people have wired
in extensions, so you need to trace the wires from every device on the
phone line to make sure they all end up on the phone side of a filter.
