Good advice for anyone starting out. Personally, again through a lot of trial and error (and a good amount of sucking back and thinking critically about this problem set), I have found, as a hard-gainer/true ectomorph, that HIT works best for me. Hit the muscles hard in all-out effort and then rest. High volume regimes worked for a few weeks and then nothing. Again, to each his/her own.
I am not discounting P90X or CrossFit, as these regimes are a welcome step in creating functional muscle and muscle memory, as strength/size should not be the sole focus. As someone used to HIT, and with the knowledge that high-volume training mixed with higher than recommended intensity (potential conflict of first principles) was just too much for my particular recovery ability, I would have to look at limiting the frequency of a CrossFit regime, for example. Say 2 on, 2 two off and monitor from there.
For those who like to monitor progress objectively vs your gut feel/current state of mind, mixing up a routine too often or every time could make tracking progress a pain. Hard to tell at that point whether or not overtraining is setting in or not. Again, creeps up insidiously.