The answer of Chris Jackson is correct, however, if your model has been running for hours, you will not be happy, because it means modifying the model and running it again. But you can achieve your goal in R in the post-process without running the model again - by taking mean of the posterior samples:
out <- bugs(...)
sapply(out$sims.list$mu, mean, ...) # I'm not sure exactly about the structure of
# out$sims.list$mu, so it might be slightly
# different