I've just spent a few weeks trying to get Freeview (we aren't really in the area) so I can offer some advice.
Firstly, if you haven't got one, get an outdoor aerial. You'll probably need a wideband aerial.
If you're in a fringe area, I can recommend Televes aerials. They look wierd, but it certainly improved my reception.
Replace any existing coaxial cable (from aerial to TV) with foil shielded, low loss type. Usually to FT100(?) standards, or even FT125. Ordinary TV coax is no good for digital.
Add a low noise, shielded masthead amplifier.
Or, just pay a good aerial fitter to come out and do it all for you. It'll probably cost �100+ for a proper job.