For general rules please refer to http://www.ysnz.cn/Show.asp?ArticleID=468
General rules:
上声的变调
上声在跟上声相连或跟别的声调相连的时候，都要念变调。
1、念半上──上声在阴平、阳平、去声前面念半上，调值由214变成21或211，也就是只降不升，由于上声的起音就低，所以近似低平调。例如：
每天měitiān 每年měinián 每月měiyuè
2、念直上，像阳平一样──上声跟上声相连，前面的上声变成升调，跟阳平一样（或近似阳平）。调值由214变成24或35。例如：
美好měihao 厂长changzhang 领导lingdao
The case of "不" is an exception. Mostly when two characters of 4th tone come together, the tone stays unchanged, for example 部队，半路，半步, etc.
There are lots of exceptions. For example, generally when a character of the 2nd tone follows a 4th tone character in a word , for example 布局, the tone of the second one stays unchanged. But in the case of 事情，the tone of the second one is weakened, being pronounced like a weak tone. Similar cases are 妈妈，爸爸...
