Best ways to avoid over/under-training

I am 23
215 lbs
been workin out for 5 years (3 serious)
21.23% BF
My diet is mostly milk/rice/whey/eggs/potatoes/pasta/chicken/some veggies
My goal is to gain as much muscle as possible first of all, and secondly to get as strong as possible.
I am on a 5 day split
mon-chest
tue-back
wed-legs
thu-shoulders
fri-arms(weak points/whatever I feel like doin)
sat-off
sun-off

what do you think are some of the most efficiant ways to avoid over-training/under-training?
Thanks.

I would put a rest day or two in there somewhere if at all possible

See my board for responses to your inquiry there. :)

~SC~