Mainly for noobs but may be helpful to others.

Don't hesitate for a moment once level 80 is full
Don't fall in love with weapons or camo they come back
Always check every challenge for those close to 10k
Use 2 or 3 weapons so they can achieve the 10k bonuses
Play the game type you feel best at, S+D isn't for all
Use multiple kill streak types for more bonuses
Hope this helps at least a few people