Laying long straight track accuratly

The introduction of the Google Maps overlay is a great help when creating original routes. But I have found that its almost impossible to lay a long length of straight track accurately. Trying to adjust the bearing over say a 3 mile stretch is almost impossible, especially as there seems to be limit to length of a single section of track.
Has anybody found a trick to do this? I am currently about 2 miles into a 3 mile straight and the line has drifted about 20m off, the only way to correct is to try and introduce a very subtle bend but even at the maximum radius its still noticeably not dead straight.
A method of entering start/end coordinates would be a good solution...\
Geoff
Has anybody found a trick to do this? I am currently about 2 miles into a 3 mile straight and the line has drifted about 20m off, the only way to correct is to try and introduce a very subtle bend but even at the maximum radius its still noticeably not dead straight.
A method of entering start/end coordinates would be a good solution...\
Geoff