Use different address and different account. Mix the coins if you want to get extra privacy.
For mixing service , I just know bitmixer only.
Well,if there are such services(I just saw their website) , doesn't it mean there is probably 'some' information that makes the receiver know its the same sender?
i think it's done random on their server, they can't control it, but bears in mind that even with a mixer you're not 100% anonymous