While 99.999% of the time, just using the the 3E PHB is fine, I've recently taken to building characters (Traveller Characters mind you) using my own home-brew spin on D20 Modern. It goes something like this:
1. Players pick a "base class" to start with. In d20 Modern, its the standard classes from there.. Strong, Tough, Smart, etc. This represents the overall type of person the character is. They get a free level in whatever base class they take at maximum ability (Max Hit Points for 1st level).
2. Then I have a mildly complex "career" system which at various points in the process allows them to pick up one more levels in this base class or add a new class (the basic T20 classes), and/or level up..
This allows older characters to come out of the process at a higher overall character level and a bit more well-rounded (for me anyway)..