Can Precision-Recall be improved for imbalanced sample?
I am not sure if you are asking how to improve the power of the models or how to better represent the effectiveness of the models on the minority class.  These are information retrieval measures that are sometimes adequate in other circumstances.  If you are working on an information retrieval process, this is probably fine.  If it is more of a statistical analysis you are working on, perhaps sensitivity (which is the same as recall) and specificity would be better metrics to describe your results.  Maybe your model is useful in that it can rule out the negative. These metrics can also be evaluated relative to the prior probability of the minor class in the data.  

Sensitivity and specificity have an additional benefit if you are trying to communicate your results with an audience that reviews a lot of medical test results as these measures are used most frequently in that field.

HTH