One tip - while you definitely should consider age and potential when valuing players, don't overvalue them. In both FBB and FFB, I see new dynasty owners putting too much stock in getting young guys or guys with potential way too early. Consider how rare it is for guys to turn into 1st-round fantasy players, how many guys come out each year with top potential, and how many guys end up being busts or fantasy afterthoughts.
When you load up on all young guys way too early instead of taking some top guys available, you're making it such a huge long shot that more than a couple of your guys pan out. Even if you're going to begin thinking about playing for 2009 or 2010, the more value you can get on your team the better. And if nothing else those established stars can fetch some good draft picks or young keepers from playoff teams come trade deadline.