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Be aware of JOSM shortcuts
JOSM has lots of keyboard shortcuts. Most of these can be changed (from their default) to different keys in the JOSM 'Preferences'. Shortcut lists:
I tried to download my town/city/region - why doesn't it work?
Chances are the area you tried to select is too large and the server probably timed out before getting the data to you. In order to conserve OSM's bandwidth, you cannot download or select to edit an area larger than 0.3 degrees in either dimension through the web interface. For editing, it is recommended to work on smaller areas.
If you really want larger areas of data, the best approach would be to download planet.osm or an extract. These are snapshots of the OSM database for the entire planet or a specific area.
I want to create a very long way - how do I download OSM data for such a big area?
In order to be able to easily handle long roads, you should not make one long road out of it. You should rather split the road into several ways. As a rule of thumb, no way should be longer than 10-15 km. Typically, they will actually be much shorter.
Applications like route planners for example will be able to easily join the ways to one road again. This type of application will need to postprocess the OSM data anyway.
For motorways for example, it makes sense to make a way from one exit to the next.
See also 'josm' tag on help.openstreetmap.org which has nearly 300 questions
See also JOSM wiki help page at https://josm.openstreetmap.de/wiki/Help