I tend to focus first on developing a cool character concept, history and persona - and then I min/max it to be as optimal as possible. Most of the time, that means 2 base classes and 1+ PrC's (planned, at least). Sometimes, it just means single classing with a few well selected feats. Sometimes, it's a horrible almagation of 5 or 6 classes total. It all depends on the character.
The nice thing about doing it this way is I get the heart of the character done first, and then worry about the mechanics. Not only that, but I find that the character ends up a lot cooler once I think about how to optimize him, since I'm willing to consider non-standard/traditional builds and combat styles. My favorite character ended up this way - originally meant to be a straightforward fighter/rogue, he turned into an amazingly entertaining swashbuckler/rogue/invisible blade that used TWF and daggers to suprising effectiveness - and amazing style.