For quick jobs, try Pearl Drops Tooth Polish. Very mild abrasive, so no worries about causing harm to the parts. No stains if you get it on blued parts. Re-use until you get the desired effect.
If Pearl Drops isn't available, look for toothpastes with mild abrasives - many don't have the gritty feel of PD, so you may have to try a few brands.
Doing this by hand, or do you have a buffing wheel? Be careful with the motorized wheels - easy to over-due it!