Uploading your changes to OSM
Most editors will prompt you to enter changeset comment:
- JOSM among many tools on changesets, additionally exposes manual handling of conflicts
- If you haven't uploaded your data longer than several hours or days, you may want to update your data using update tools Update Data Update selection Update modified to lower chance of conflict. You cannot avoid conflict if data modified by you was quickly altered by someone else before you uploaded your changes.
- (optional) more materials on conflict handling in JOSM:  
How to avoid conflicts (optional)
This is tricky question since if you are the only person who edits some area you may never see one. But in dense cities conflicts may occur more often due to many users editing same data at once.
- Use a real or imagery Cake diagram to avoid conflicts with other mappers
- Reduce time download-upload window
- By using smaller changesets.
- By submitting them more often than usual
- Consider submitting regular change-sets if conflicts no longer an issue