pay close attention to your appearance and manners.a smart appearance and good manners will leave a positive first apprssion.这里appearance为什么一开始不可数，后面又可数了啊，谁能解释下啊？

前后两句中的appearance都是可数的，是单数名词。一个单数的可数名词前有物主代词时是不要再用a/an 的，如your father,his mother等。

补充：据《THE ADVANCED LEARNER'S DICTIONARY》(ALD)，appearance无论解作act of appearing（出现，到场）还是that which shows or can be seen;what sth.or sb.appears to be（外观，外表），都是[C]（即作可数名词用）。而据《LONGMAN DICTIONARY OF CONTEMPORARY ENGLISH》(LD)，解作the act of appearing,as to the eye,mind or public（在眼前、脑中、公众中出现）时，是[C],而解作that which can be seen,outward qualities,look（外观，外表）时，是[U;C usu.sing]（用作不可数名词，或单数可数名词）。由此可知，appearance一词一般是当作可数名词用的。以下见于各词典的例句可以证明，appearance可用作可数名词：
I don't want to go to the garden party but I'd better put in an appearance.(ALD)
We mustn't judge by appearances.(ALD)
My appearance at the party was not very welcome.(LD)
After being painted its appearance was quite different.(LD)
Appearance isn't everything in life,my boy.(LD)
There is every appearance of rain. （《新英汉词典》）
Don't judge by appearances. （《新英汉词典》）
Never judge by appearances.(《A PRACTICAL ENGLISH-CHINESE DICTIONARY》)
Did you have any wound before the appearance of this tumor? (《A PRACTICAL ENGLISH-CHINESE DICTIONARY》)
From his appearance he seemed very wealthy.(《A MODERN ENGLISH》)
She had a sad appearance.(《A MODERN ENGLISH》)
His sudden appearance supprised her.(《A MODERN ENGLISH》)
此外，短语by appearances(根据外表),make/enter an appearance(到场),by all apearances(显然），keep up appearances（装门面），save appearances(保全面子)等都表明，appearance是经常用作可数名词的。
至于问题中的前一个appearance，意为“外表”，当然可以且应当看作可数名词。楼主之所以感到困惑，愚以为是由于其后的manners的困扰所致：manners作“礼貌”解时是只能用复数式的，这样就可知，其前的your未必要解为“你们的”，实际上，这个your就应该或者说只能解为“你的”！your appearance 解为“你的外表”，当然用单数！（这里没有什么“抽象概念具体化”之类的问题。）