The gentlest way of cleaning metal is using washing soda and aluminium foil, and it's dead easy!
Put a piece of foil (ordinary kitchen aluminium foil) in the bottom of a plastic bowl. Add had-hot water and throw in a handful of basic washing soda (you can get it from most supermarkets). Swirl the soda to dissolve. Now put your items to clean in the solution, making sure it touches the foil - tarnish will almost drop off anywhere in the water. A gently rub will help, and you can use a soft toothbrush to get into awkward bits. Give it a rinse in clean water, and dry/buff with a soft cloth - job done!
This has worked on every piece of jewellry I've tried it on, and I used to clean all the trophies at school this way, as it's so easy and leaves no polish residue in corners!