You get titles and emblems for doing the majority of challenges you see.
I f you look through your challenges you will find that the challenge names are on the titles, some times.
there are many apps on the IPhone and I Pod touch that tell you how to get the titles and emblems.
I hope this is a good answer for you
to get titles and emblems on modern warfare 2, you have to complete the challenges in the barracks, but for some you have to prestige
No you don't lose your emblems and titles
you unlock it by doing multiplayer it just matters how much kills, or falling, and random things you do
No unfortunately same as you can't level up in private matches
Yes you can
No, all your guns are reset to the same as you were when you were level 1. The only thing you unlock when prestiging is emblems and titles.
Complete challenges and level up.
Your face thats the answer
You gain titles and emblems by completing certain challenges on multiplayer, check your barracks for the challanges. Here is a list of all the titles and emblems for call of duty modern warfare 2 on the Mordern warfare wiki: http://callofduty.wikia.com/wiki/Callsigns
You need to be more specific on the "Diamond" there are a few emblems that look like Diamonds Here is a link to the list of emblems and how to get them: http://emotionalninja.net/modern-warfare-2-how-to-get-titles-and-emblems/776/2
Get to 10th prestige, unlock everything (challenges, emblems, titles, weapon challenges and camos, etc....)
Because, you will actually gain more titles, emblems, and challenges. Why not?