sorting factors

Hi,

I've been trying to sort the values of the factors of contingency tables 
generated with xtabs. For example, I have a factor called "artic" with three 
possible values that I would like to order in a specific way -- but I'm not 
sure how to go about this.

test <- read.table("test.tab",header=TRUE,sep="\t")
artic = factor(artic,levels=c("lowlip","dental","apex"))
test.frame <- as.data.frame(test)
test.xtabs <- xtabs(freq ~ artic + var, test.frame)
mosaicplot(test.xtabs,shade=TRUE)

Any help appreciated,
Gina