..often needed when preparing data for analysis (and usually forgotten until I need it for the next time).
With the below code I convert a set of variables to factors – it could be that there are slicker ways to do it (if you know one let me know!)

> dat <- data.frame(matrix(sample(1:40), 4, 10, dimnames = list(1:4, LETTERS[1:10])))
> str(dat)
'data.frame':   4 obs. of  10 variables:
 $ A: int  5 34 3 15
 $ B: int  28 25 17 24
 $ C: int  2 12 10 32
 $ D: int  16 27 29 14
 $ E: int  40 7 4 31
 $ F: int  22 30 6 18
 $ G: int  33 36 35 38
 $ H: int  19 21 37 8
 $ I: int  20 11 9 26
 $ J: int  39 13 1 23
> 
> id <- which(names(dat)%in%c("A", "F", "I"))
> dat[, id] <- lapply(dat[, id], as.factor)
> str(dat[, id])
'data.frame':   4 obs. of  3 variables:
 $ A: Factor w/ 4 levels "3","5","15","34": 2 4 1 3
 $ F: Factor w/ 4 levels "6","18","22",..: 3 4 1 2
 $ I: Factor w/ 4 levels "9","11","20",..: 3 2 1 4