With digital its all about the SNR (signal to noise ratio).....you want more signal and less noise. If you boost/amplify this then quite often the noise will increase, and the SNR will actually go down, which is bad.
However, a decent masthead amplifier installed correctly (although aimed at proper 'outdoor' aerials) should help improve things, as much of the noise is avoided due to the amp being installed as close to the aerial as possible.
Also, use shielded satellite cable where possible to prevent noise introduction into the system.
antony, yes digital doesnt have that much power allocated to it at the moment, but thats simply because it cant, as analogue still takes priority. Digital covers about 80% of the country, and with a decent aerial installation, nearly all these people should be able to pick it up.
We're in an area where Freeview is apparently not available, but I installed a wideband aerial, masthead amplifier and shielded cabling and am now able to pick up 90% of the available channels.
Edit: Sky is rubbish, most of the channels are tosh.