[Help] Criteria for choosing # of clusters in hierarchical clustering

[Fred]

Dear all, I am now working on the hierarchical clustering methods, and confused about the following problem. As you know, to form clustering from the hierarchical tree generated by the pairwise distance bw the elements, we have to set a threshold value to cut the tree horizonally such that the vertical links intersecting with this horizonal critical value will be the final clusters. However, I do not find a very robust criterion for choosing the optimal number of clusters or calculating this threshold value to make the clustering results good different pairwise distance(similairty) measure.  So any one has some point on this problem or recommended papers or methods? Thanks for your help. Fred