You could specify your sources in the changeset comments, to make it clear where your data is coming from. Or add source tags, to the changeset or to the objects.
Also you could try using the "merge" tool in JOSM, to combine the duplicate nodes, keeping the relevant tags from each. Note you can hold down CTRL and drag one node on top of another to merge it.

It would also be a good idea to use the standard tagging for collection times. See osm.wiki/Key:collection_times

I think a tag for the box type is a good idea, but I'm not sure if there is any standard tag agreed for this. Maybe worth proposing something and documenting it on the wiki.
And a tag for the royal cypher as well?

Try a filter: "-amenity=postbox"
That will filter everything that is not tagged as amenity=postbox.

Or for what you want, something like:
-(amenity=post_box new_upload=yes ref=SG*)

It looks like someone has done a bulk import of government data for land use in Georgia, but not Atlanta (or any other states?).
Its mentioned on this page: osm.wiki/Georgia_%28U.S._state%29#Land_Use

I don't know whether any similar data is available for Alabama or elsewhere, or how useful it is.

For a road with multiple arms, yes you just draw it as multiple ways, all branching off each other. And tag them all with the same name= etc.
There's no need to use a relation for this - if the ways are all connected to each other, its obvious its all part of the same road.
Mapnik and Osmarender can render these fine.

There is some phone software that does rendering of vector maps from the OSM data. eg GpsMid http://gpsmid.sourceforge.net/
Or other options here, the list specifies which do vector maps: osm.wiki/Software/Mobile_phones

Also worth comparing your GPS tracks with the OS Streetview maps - they are pretty accurate in most places. Much better than NPE maps anyway!

Though surveying on the ground is still a good idea to check its correct, plus you can find lots of stuff that isn't shown on the OS maps, eg postboxes, bus stops etc.

Rivers should be mapped as an area (tagged waterway=riverbank) and a way (tagged waterway=river). See osm.wiki/Tag:waterway%3Driverbank

Its not just for rendering, it also indicates the direction the river is flowing. And it could also be used for routing, or measuring the length of a river etc.

Seems like a bug, it's been reported here:http://trac.openstreetmap.org/ticket/2514
Though nothing seems to have happened with that for months. I though it would be a fairly simple fix if streams and canals etc are named correctly.

I think it would help make things easier for the renderers (and other data users) if it was a separate tag (and it would be more backwards compatible).

Otherwise, how do you tell the difference between a drain that loops back on itself (so is a circular way), and a drain mapped as an area? You could use area=yes/area=no, but that's an additional tag to remember.
A different tag also means validators can warn of problems, as they do for non-closed areas for riverbanks etc.
Also, renderers could ignore riverbank areas etc, and just render the river as a line at lower zoom levels, or to make a simpler map.

In Scotland, it is legal to walk, cycle, ride a horse etc, just about anywhere (so long as you are responsible). See the Scottish Outdoor Access Code.

The wiki definition of highway=footway says "For designated footpaths, i.e. mainly/exclusively for pedestrians".
As there are not many paths in the countryside exclusively for pedestrians, I think its usually best to use highway=path. Plus add the appropriate access tags, ie foot=yes, bicycle=yes, horse=yes.
And its also a good idea to tag the surface=, and for mountain paths / hiking trails, you can also tag with sac_scale= and mtb:scale= etc.

For drains, is it worth mapping them as a way and an area, as is done with rivers (waterway=river/waterway=riverbank)?
eg waterway=drain (for a way) and waterway=drainbank (for the area). It means you can indicate the direction of flow.

The same thing applies to streams. Streams as areas are available from the OS Opendata vector maps, so it would be nice to map them in OSM.

@ Sanderd17: I presume the rendering of Johannesburg is different to London because it is not the capital. I think places tagged as place=city and capital=yes render at lower zoom levels.
As it is, Johannesburg renders the same as the surrounding cities. Maybe it would be useful to have a tag for big/important cities, that are not (national) capitals. Or tag it with the population, and render based on that.

The tag highway=incline_steep or highway=incline is intended to be used on nodes, not on the way (which should have the usual highway=path tag etc).
Though its not very useful on nodes, as it doesn't indicate the direction of the incline. So its now discouraged / deprecated.

Its better to tag the way with incline=up or incline=down, or incline=10% etc.

Also worth tagging the paths with the surface: osm.wiki/Key:surface
Its useful to know whether its the path is paved or gravel or dirt etc (though I'm not sure if any renderers use this).
And if its a hiking trail you can tag with SAC scale: osm.wiki/Key:sac_scale

Go to this webpage and pick the tiles you want: http://garmin.na1400.info/routable.php
The map tiles will be a bit bigger than just Crete, but it shouldn't take too long to download.

Might be more useful to use something like old_name:de to specify what language the historic name is in.

Mapsource lets you view the maps on your PC. You can do stuff like plot waypoints and routes, then send them to your Garmin. Or download the tracklog from your Garmin and view it on a map, plus you can split/join/edit tracks. Then save it as GPX format, and upload it to OSM if you want.

So I find it very useful for doing this sort of thing, using it with my eTrex. I'm not sure if your Nuvi supports uploading/downloading waypoints etc?

Also, you can select maps and send it the Garmin. So you could download OSM maps for the whole of the USA etc, then install into MapSource. Then you can just select some of those, and send them to your Garmin if you don't have much space on it.

Though note MapSource usually included with the GPS device, or with maps you buy from Garmin. Updated versions of MapSource are available to download for free from Garmin, but they won't install unless you have a previous version installed. Unless you use a bit of workaround, see the instructions here: http://www.mydigitallife.info/2008/01/31/download-garmin-mapsource-for-free-and-install-without-media/

As for the file sizes you mention, I think they are a bit mixed up. I suspect you mean 8MB, not GB. Though I think it would be easiest to just buy a 2GB or 4GB SD card, they are pretty cheap now. Then you can copy the OSM maps onto that, and not affect the preinstalled Garmin maps.

Assuming you are talking about this website: http://garmin.na1400.info/routable.php

It says on that page;
This service generates four different configurations of your personal map:
* Installer for Garmin MapSource (Windows).
* Installer for Garmin RoadTrip (Mac OSX)
* Combined image for direct manual placement on the GPS device (gmapsupp.img)
* A zip file just containing all the Garmin map tiles as selected on this website. This is useful for Linux users (e.g. Qlandkarte)

So these files are useful if you want to view the map on your PC, in different software.

Have you seen the table for international equivalence at the bottom of this page: osm.wiki/Key:highway
It specifies what type of roads should have what tags for each country.

Have you followed all of the import guidelines? osm.wiki/Import/Guidelines

Specifically, discussing it with the community, before you import anything. And documenting it on the wiki.
Also, how do you know that your "official" data is better than what was there previously?

I think there's a few problems with how Opencyclemap handles multipolygon relations for areas. It seems it applies the tags on the inner part to the whole polygon.
I think its the bug reported here: http://trac.openstreetmap.org/ticket/1656