No, save each little change that you make, together with a comment. Its in your own best interests to make lots of small commits - that way, you get your work saved a.s.a.p.

You must be doing something wrong. I'm only a newbie (April 2010) but this might help: Save your tracklog(s) from the GPS-device as a .gpx file. Double-click the .gpx file which opens JOSM. You see the tracklog as a grey line on a black rectangle. Centre it up and zoom using mouse scroll wheel. Do menu "File/Download from OSM...". The black rectangle (hence your zoom level) decides the size of the red rectangle on the slippy map, but you can also redraw the red rectangle. Once its loaded, do your edits. Now when you save via the menu "File/Upload data" only your edits get uploaded, and this is usually a tiny amount of data and should take only a matter of seconds with a 5 KB/s modem.

Tracing from proprietary maps provides a starting point, but isn't it frowned upon? All through OSM Wiki they keep saying "Don't copy from other maps". So if I were you, I'd go out with the unit and walk those woodland boundaries and correct them in OSM.

Reading glasses! This is openstreetmap. No-one affluent here.