There are times you just have too much data, random samples are nice to test assumptions and algorithms first.

So in R you can create a function to return a random sample of a data frame for such emergencies.

randomSample = function(df,n) { 
   return (df[sample(nrow(df), n),])

And to use:

smallerDF<-randomSample(bigDF, 40)

(40 being the number of rows you want in your sample).